Standby Generator Maintenance Explained: The Complete Schedule for Australian Diesel Gensets

If you own or manage a standby diesel generator — for a dairy shed, farm, workshop or small industrial site — the single most important question isn't how big it is or what it cost. It's whether it will actually start and carry load the one time you genuinely need it. That comes down entirely to maintenance, and standby generators fail differently to almost any other piece of equipment you own: they can sit perfectly still, looking fine, for months, while the exact things that stop them working quietly get worse in the background.

This guide pulls together manufacturer service data (Cummins Onan, Kohler, Generac, Perkins, Caterpillar, Kubota, FG Wilson, Deutz), the US NFPA 110 standard for standby power testing, Australian standards and farm-specific guidance, and genuine field experience from technicians and generator owners on forums like Mike Holt's Forum, BobIsTheOilGuy and TractorByNet. Where sources agree, we say so plainly. Where they genuinely disagree — and on a couple of real questions, they do — we tell you that too, rather than pretending there's one universal number.

Supplying the parts that keep a standby generator running is what AIMS does — batteries, fuel treatments, lubricants and consumables. Servicing the engine itself (valve clearances, injectors, electrical testing) is licensed-technician work, and we'll be upfront throughout this guide about exactly where that line sits.

Standby Generator Maintenance — Quick Reference Schedule

There's no single hour table that applies across every brand — Perkins says plainly that "no two schedules are exactly the same." But the underlying structure is remarkably consistent across Cummins Onan, Kohler, Generac, Perkins, Caterpillar, Kubota, FG Wilson and Deutz. This table shows the typical shape; treat the hour figures as indicative, not gospel for your specific model.

Two real, sourced examples of how this plays out for specific engines: Kubota (a very common engine in the 20–100kVA class sold in Australia) specifies oil and filter changes at 250 hours, valve clearance checks at 500 hours (0.18–0.22mm intake and exhaust), and full injector servicing with a compression check at 2,000 hours. FG Wilson (Perkins-engined, also common in this size range) uses a simple minor-service-at-500-hours, major-service-at-1,000-hours structure, plus a standby-specific instruction to start and run the unit for 5 minutes every two weeks regardless of hours.

Notice what both of these have in common with every other manufacturer we checked: nobody frames standby generator maintenance purely by engine hours. A generator doing genuine standby duty on a farm might only clock 20–30 hours a year — but oil, coolant and fuel all degrade chemically over calendar time as well as run time. That's why almost every schedule above says "hours or annual, whichever comes first."

How Often Should You Actually Run a Standby Generator? (The Real Debate)

This is the single most-asked question about standby generator maintenance, and honestly, it doesn't have one universally agreed answer — even among manufacturers and standards bodies. Here's the genuine spread of positions, so you can pick the one that fits your engine and your risk tolerance.

The regulatory-style position: NFPA 110

NFPA 110 is a US standard for emergency and standby power systems — there's no direct Australian equivalent that mandates testing intervals the way NFPA 110 does, but it's widely used internationally as a best-practice benchmark. It requires:

• Weekly: a visual inspection of the whole system — fuel, oil, coolant, battery/charger, leaks, alarms. About 15–20 minutes, aimed squarely at catching the single most common failure: a dead battery.
• Monthly: an exercise run of at least 30 continuous minutes at not less than 30% of the generator's nameplate kW rating (or enough load to reach the manufacturer's minimum exhaust gas temperature) — plus exercising the automatic transfer switch under load.
• Annual: a load-bank test, required whenever the building's actual monthly load can't reach that 30% threshold on its own — typically run as a graduated multi-step test (sources vary on exact percentages between NFPA 110 editions, generally somewhere in the 25–75% nameplate range across the steps).

The manufacturer position: it depends on your engine

Here's the genuinely interesting nuance most generic advice misses. Kohler's own published position, in a technical white paper aimed at industrial customers, is that modern diesel engines with electronic common-rail fuel injection can be exercised no-load, monthly, provided the unit gets a proper full load-bank test annually. Their reasoning: today's precision injection and piston/ring assemblies simply don't wet-stack the way older mechanically-injected engines did at light loads. In Kohler's own worked example, a large industrial genset run no-load monthly plus full-load annually used 71% less fuel than the same unit run at full load every month — with no measurable downside.

That's a real, sourced manufacturer position — and it sits in genuine tension with the older "always load it, every time" rule that still dominates a lot of forum advice and older engine folklore. The honest answer is: which one applies to you depends on whether your engine has old-style mechanical injection or modern electronic common-rail injection. If you're not sure, ask your dealer or check your model's fuel system type before deciding.

What generator owners and technicians actually do

Real-world practice, per long-running threads on Mike Holt's Forum (electricians) and BobIsTheOilGuy, is genuinely split:

• Some technicians run weekly, under real transferred load, for 20–30 minutes, and treat unloaded testing as inadequate — their logic being that a no-load run never gets the thermostat fully open or the engine properly up to temperature.
• Others — including operators managing multiple standby units across sites — run monthly, for 45–60 minutes, and consider that sufficient for modern engines.
• A recurring piece of advice worth adopting regardless of which camp you're in: a couple of times a year, do a genuine "plugs out" test — physically disconnect from mains and let the transfer switch do its real job — rather than only ever watching the generator start on a scheduled exercise cycle. A transfer switch that starts the generator on command isn't proof it will actually transfer load correctly when the grid genuinely drops out.

Battery Maintenance: The Single Most Common Reason Standby Generators Fail to Start

Every source we reviewed — manufacturer guidance, technician forums, dealer checklists — names the same top failure cause for standby generators that won't start when actually needed: a dead or weak starting battery. It's not close.

The mechanism is what makes this genuinely deceptive. A battery sitting on float/trickle charge between exercise runs degrades through sulfation — a slow chemical process that eats away at real cranking capacity while the battery can still read a perfectly normal 12.6V at rest. The battery looks fine on a voltmeter and then fails the moment it's asked to deliver real cranking current, which is exactly the moment you need it most. This is also why cold weather is a double risk: low temperatures reduce a battery's real capacity by roughly 20–40% on top of whatever sulfation has already done.

Terminal corrosion prevention is one of the cheapest, easiest wins in this entire guide. AIMS stocks CRC Battery Maintenance Spray for cleaning corroded terminals and CRC Battery Terminal Protector to stop it recurring — a five-minute job done annually that directly addresses the single most common reason standby generators don't start. For the float/trickle charging itself, the Hansa Battery Smart Charger range covers the multi-stage charging described above from 2 amp up to 12 amp, 6/12/24 volt — and the same collection carries a battery hydrometer if you're checking specific gravity on an older open-cell battery, plus jumper leads for the day the battery's already caught you out.

Fuel Storage & the "Diesel Bug": Why Standby Fuel Fails Differently

Diesel fuel doesn't sit inertly in a tank. It oxidises, absorbs water, and — given the right conditions — grows microbial contamination ("diesel bug") at the fuel-water interface. This matters enormously more for a standby generator than a generator in constant use, because a genset running regularly turns its fuel over fast enough that it rarely sits long enough to degrade. A standby unit sitting idle for months between exercise runs is exactly the scenario where this becomes a real problem — and multiple sources name stale or contaminated fuel as one of the top real-world reasons standby generators fail.

How long can diesel actually sit before it's a problem? Honestly, sources give a genuine range rather than one fixed number — reported figures run from roughly 6–12 months untreated before real risk sets in, extending to 18–24 months with active management (biocide treatment, water draining, temperature control). Older, pre-ultra-low-sulfur diesel used to last considerably longer in storage — 3–5+ years under good conditions was achievable. Modern ULSD (ultra-low-sulfur diesel) degrades faster because the same refining process that strips out sulfur for emissions compliance also removes natural stabilising compounds, while producing smaller, more reaction-prone molecules. Biodiesel blends compound this further — they hold more suspended water, which is exactly the food-and-water combination microbes need.

What causes diesel bug, specifically

Three things have to be present together: water (from condensation as temperature swings in above-ground tanks, or rain ingress), a food source (biodiesel content is an ideal microbial nutrient), and warmth (growth thrives roughly 10–40°C — most of Australia, most of the year). Once established, the resulting biomass and sludge clogs fuel filters and injectors, degrades combustion, and can cause a fuel-starvation shutdown at exactly the wrong moment.

What actually prevents it

• Drain the water separator regularly — this appears as a task on virtually every manufacturer's schedule, from daily/weekly (Perkins, FG Wilson) to every 50 hours (Kubota). It's the single highest-value, lowest-effort fuel task on this list.
• Treat fuel intended for extended storage — biocide and stabiliser additives need to go in while the fuel is still fresh; they slow degradation but can't reverse damage that's already occurred. One Australian generator dealer specifically recommends biocide and cetane booster treatment for any diesel expected to sit more than three months — treat this as one supplier's stated practice rather than a universal figure, but it's a sensible rule of thumb.
• Keep tanks topped up where practical — less empty headspace means less room for condensation to form as temperatures cycle day to night.
• Fuel polishing — recirculating stored fuel through filtration and water separation to recondition it in place — is worth considering for larger or ageing fuel reserves, particularly if you're not confident about how long fuel has been sitting.

For the fuel storage side of this — tank sizing, bunding, and what AS 1940 actually requires for storing diesel safely on-site — see our dedicated Diesel Fuel Storage in Australia guide. AIMS also stocks Morey's Diesel Smoke Killer, a diesel fuel treatment that addresses combustion-related smoke and residue.

Cooling System Maintenance

A 50/50 mix of ethylene glycol coolant and clean or de-ionised water is the standard baseline — it gives freeze protection to around −34°C, raises the boiling point, and includes corrosion inhibitors that plain water or unmixed antifreeze simply don't provide. Coolant should be tested annually (nitrite/SCA levels for older-style coolant, or the manufacturer's specified test for extended-life coolant) and fully replaced on a calendar or hour-based schedule depending on how the unit is used — every 3–5 years is a reasonable default for a lightly-used standby unit, though high-hour industrial units often work to a specific hour interval instead (Caterpillar's larger industrial engines, for example, specify extended-life coolant replacement at 6,000 hours).

Hose inspection for leaks, cracking or restriction is worth doing quarterly, and vibration from loose mounting is a genuine cause of stress cracks at hose and tube joints over time — something to check if a unit is mounted on a skid or trailer rather than a fixed concrete pad. AIMS stocks radiator hose and radiator overflow hose if yours needs replacing rather than just topping up. Coastal or humid sites face accelerated external corrosion of the cooling package; if your generator lives near the coast, galvanised or powder-coated components and stainless fasteners are worth specifying when you're next replacing hardware.

Air Filtration: Why Australian Conditions Change the Rules

The general pattern across manufacturers is: inspect the air filter around every 50 hours, replace the primary element somewhere in the 250–500 hour range (or annually), and replace the inner "safety" element less often again. One thing worth correcting directly: diesel air filter elements should be replaced, not cleaned and reused — that "clean it out and put it back" habit is more valid for older oil-bath or foam-element small engines, not the paper-element filters on modern diesel gensets.

Deutz's own Australian guidance is blunt about this: equipment "used in harsh conditions, such as dusty or humid environments, may require more frequent servicing to prevent contamination and wear." For a generator sitting in an open-sided farm shed through an Australian summer, that's not a hypothetical — one Australian dealer source specifically recommends halving the typical air filter interval to around 250 hours in hot, dusty conditions. If your generator lives somewhere genuinely dusty, build that shortened interval into your own schedule rather than defaulting to the manufacturer's baseline figure, which usually assumes a cleaner operating environment than a working farm or industrial yard.

Oil and Oil Filter Service

Break-in service typically falls in the first 20–50 hours across brands. After that, 250 hours is the most commonly cited standard interval for oil and filter changes, though several manufacturers extend this to 500 hours or annual — whichever comes first — for standard-duty standby use rather than continuous heavy-duty running. What actually governs the real interval: engine size and sump capacity (larger, deep-sump engines tolerate longer intervals for the same oil volume relative to blow-by), duty cycle (continuous/heavy duty shortens the interval), and ambient conditions (hot, dusty environments shorten it further, per the same logic as air filters above).

Larger commercial and fleet operators sometimes run oil analysis programs — Caterpillar's S·O·S Scheduled Oil Sampling program is the best-known example, testing oil, coolant and fuel samples to catch developing wear before it becomes a failure, and interestingly around a quarter of the samples Cat's labs process come from non-Cat equipment, confirming it's used as a general condition-monitoring tool rather than a Cat-only program. For most farm or small-industrial operators running a single standby unit, this level of program is genuinely more than you need — it's a fleet-management tool for people running dozens of units, not something worth setting up for one 40kVA shed generator. Straightforward calendar/hour-based oil changes, done on time, get you the vast majority of the benefit.

Valve Clearance & Injector Service: Technician-Only Work

Larger service intervals — valve clearance adjustment and injector servicing — sit at genuinely different hour marks depending on engine size: Kubota specifies a valve clearance check at 500 hours and full injector servicing with a compression test at 2,000 hours; Cummins Onan calls for valve lash adjustment at 800–1,000 hours; larger Caterpillar industrial engines push both out to 4,000 hours, reflecting their heavier-duty construction.

Every source we checked is consistent and unambiguous on one point: this is qualified-technician work, not an owner/operator task. Cummins Onan's own manual states plainly that valve lash adjustment "must be performed by a trained and experienced mechanic." It requires specialised tools (feeler gauges to exact spec, injector test benches, correct torque sequences) and getting it wrong risks real engine damage or voiding your warranty. AIMS's role here is straightforward: we're the source for the parts your technician needs — filters, gaskets, belts and consumables — not a substitute for the qualified diesel mechanic or dealer network who should be doing this specific work.

Belts

Older V-belts should be discarded if you see any horizontal or vertical cracking. Newer serpentine/multi-V belts are more tolerant — horizontal cracks along the ribs are generally acceptable unless they reach into the belt's backbone, but any vertical crack between ribs means replace it now. Belt tension and condition checks belong at the daily/weekly visual-inspection level — genuinely something an owner or site operator can and should do themselves, unlike the valve and injector work above. There's no single universal replacement interval; the honest answer from the sources we reviewed is "replace when your manufacturer's schedule calls for it," though Cummins Onan does specify a longer 5-year/2,000-hour cycle for belt and bearing replacement together on its smaller diesel units. AIMS stocks a full range of Industrial V-Belts if yours is due — take the old belt in for a size match, or check the profile and length stamped on the belt itself.

Control Panel & Transfer Switch: Part of the Same Routine

Your automatic transfer switch (ATS) needs the same "test it before you need it" logic as the generator itself — it sits stationary waiting to act, and dust, moisture, insects and corrosion can silently degrade it in exactly the same way sulfation quietly degrades a battery. NFPA 110's monthly exercise requirement explicitly includes exercising the ATS under load, not just starting the generator. A sensible routine: exercise the ATS as part of your regular generator run, do a closer inspection annually (wiring connections, switching contacts, indicator lights), and do a genuine "plugs out" real-transfer test a couple of times a year, not just a scheduled exercise cycle. For the full detail on choosing and sizing a changeover switch in the first place, see our Manual vs Automatic Changeover Switch guide.

The Real Reasons Standby Generators Fail (Ranked by How Often)

Pulled from technician field data and forum-reported real-world experience, not textbook theory — this is roughly the order in which standby generators actually let people down:

1. Dead or weak starting battery — see above; consistently the single most common cause across every source.
2. Stale or contaminated fuel, clogged filters — the standby-specific fuel problem covered above.
3. Start switch left in OFF instead of AUTO — a genuinely mundane human-factor failure, not a mechanical one, but real enough that it makes several professional "top causes" lists.
4. Low coolant / overheating shutdown — usually from a slow leak (worn hose, failing pump) that goes unnoticed between checks.
5. Wet stacking from under-loading — see our Load Bank Testing guide for the full mechanics.
6. Control panel or sensor/wiring faults — outdoor-sited units are particularly exposed to moisture ingress affecting the automatic start sequence.
7. Air intake or exhaust blockages — dirty filters, or on rural sites, genuinely just leaves, dirt or pest nesting restricting airflow into an open shed-mounted unit.
8. General deferred maintenance — skipped oil changes, filters and belt checks compounding over time into a bigger failure.

Worth noticing: the top two causes — battery and fuel — are also the two cheapest and easiest to prevent. A weekly glance at the battery terminals and voltage, and keeping on top of fuel condition, addresses the majority of real-world standby generator failures before they happen.

Australian Standards, Farm-Specific Guidance & Climate Considerations

Australia has no direct equivalent to NFPA 110 mandating standby generator testing intervals. What does exist:

• AS/NZS 3010 (Electrical installations — Generating sets) covers installation and electrical safety — earthing arrangements and changeover switching — not an engine maintenance schedule. It does carry ongoing RCD testing obligations (monthly push-button test, annual trip-time test) as part of general electrical safety compliance.
• AS 1940 governs safe storage of diesel fuel (tank bunding, fire rating) — see our separate Diesel Fuel Storage guide for the full detail.
• No Clean Energy Council standard or WorkSafe regulation specifically mandates standby diesel generator maintenance intervals — CEC's remit is overwhelmingly renewables, and state electrical safety guidance focuses on installation/connection safety rather than ongoing service scheduling.

For dairy and farm operators specifically, Agriculture Victoria's Energy Smart Farming program has genuinely useful, Australia-specific guidance: backup power is described as critical for milk cooling, milking schedules and animal welfare, with a recommendation to hold a minimum 2–3 days of fuel on-site and to size the generator against your milking plant's actual load profile with a licensed electrician. Interestingly, poultry operations are specifically advised to run standby generators at least fortnightly — a real, citable Australian primary-production testing cadence, even though it's sector-specific rather than a universal standard.

Two climate factors are worth building into your own schedule regardless of what the manufacturer's baseline assumes: heat and dust shorten air filter and oil intervals in most of rural Australia (see above), and coastal or humid sites accelerate corrosion of the cooling package and cabinet — galvanised, powder-coated or stainless componentry is worth the extra cost near the coast.

Building Your Own Maintenance Routine — A Practical Approach

If you take one thing from this guide, make it this: a documented, followed schedule — even a simple one — beats an elaborate schedule that gets skipped. Here's a practical routine that reflects the research above, scaled for a single standby unit rather than a commercial fleet.

1. Do a weekly visual and battery check: a walk-around (fluids, leaks, belts), battery terminal and voltage check, and an exercise run — ideally under some real load, especially if you're not sure of your engine's injection type.
2. Load-test the battery every quarter: not just a voltage check, inspect and clean battery terminals if needed, and drain fuel/water separator sediment at the same time.
3. Service filters and fluids at the manufacturer's interval: oil and filter change, fuel filter replacement, air filter service (shortened if your site is dusty), coolant condition check, and belt inspection, at the hour or annual mark, whichever comes first.
4. Book an annual load-bank test and ATS check: a technician-run full load-bank test, deeper coolant and electrical checks, and an ATS inspection, plus at least one genuine "plugs out" transfer test done yourself.
5. Schedule technician-only valve and injector work: at the intervals your engine specifies (typically 500–4,000 hours), booked with a qualified diesel technician, not attempted on-site.
6. Keep a written maintenance log: date, hours, what was done, by whom, every time. A simple logbook is genuinely enough; the value isn't the format, it's having a record that shows whether your generator has actually been looked after when you need to prove it has.

Generator Maintenance Parts at AIMS Industrial

AIMS stocks the consumables that keep standby generators reliable between technician services — battery care, cooling hoses and Industrial V-Belts among them — alongside the buying guides that cover the rest of this vertical: generator sizing, standby vs prime engine ratings, changeover switches, PTO-driven generators, and load bank testing.

Shop Battery & Fuel Maintenance →

Frequently Asked Questions

How often should I run/exercise a standby generator?

There's genuine disagreement here, so the honest answer is a range rather than one number. NFPA 110 (the US standby-power standard, used internationally as a benchmark) requires monthly running at 30 minutes minimum, at not less than 30% of nameplate load. Kohler's own technical guidance says modern electronically-injected diesel engines can be exercised no-load, monthly, provided you get an annual load-bank test — a real manufacturer position, not folklore. Real-world technicians on forums split between weekly loaded runs and monthly runs. If you're unsure which applies to your engine, default to running it monthly under real load, and get an annual load-bank test regardless.

Why won't my generator start after sitting unused for a long time?

In order of how often each cause actually turns out to be the problem: a dead or weak battery (sulfation degrades cranking capacity without changing the resting voltage much, so it can look fine and still fail to crank), stale or contaminated fuel clogging the filter, the start switch left in OFF instead of AUTO, or — on smaller petrol units specifically — gummed carburettor components from degraded fuel. Check the battery and fuel first; they're the two most common causes by a wide margin.

How long can diesel fuel sit in a generator's tank before it goes bad?

There's no single agreed figure — sources give a genuine range of roughly 6–12 months untreated before real risk sets in, extending to 18–24 months with active management (biocide treatment, regular water draining, temperature control). Modern ultra-low-sulfur diesel degrades faster than older diesel because the refining process that removes sulfur also strips out natural stabilising compounds. If your generator is genuinely standby-only and the tank might sit for extended periods, treating the fuel and draining water regularly matters more than it would for a generator in daily use.

What does a standby generator service actually include, and can I do it myself?

There's a fairly clear DIY-versus-technician line across every source we checked. Owner-appropriate tasks: fluid level checks, visual leak/belt/hose inspection, battery terminal cleaning, air filter inspection, and draining the fuel/water separator. Technician-only tasks: valve clearance adjustment, injector servicing, coolant system flushing and pressure testing, and ATS internal inspection — these need specialised tools and getting them wrong risks real engine damage or voiding your warranty.

Why do generators need to be run under load, not just started?

Running unloaded or lightly loaded doesn't get the engine hot enough for complete combustion. Unburned fuel residue builds up and fouls injectors, valves and cylinder walls — this is wet stacking. It's the reason exercise-cycle load level is such a live debate (see above): the risk is real for older mechanically-injected engines, and lower but not zero for modern electronic engines run persistently unloaded.

How often should the oil be changed in a generator that barely gets used?

Whichever comes first: the hour interval (commonly 250 hours for standard duty, though it varies by engine and manufacturer) or a calendar interval, usually annual. Oil degrades chemically over time even sitting completely unused, which is why almost every manufacturer schedule specifies "hours or annual" rather than hours alone.

How long do generator starting batteries last, and should I replace before it fails?

Most sources converge on 2–3 years as a proactive replacement interval, regardless of how the battery tests at the time — sulfation damage isn't reliably visible on a simple voltage check, so waiting for an obvious failure means waiting for the worst possible moment to find out.

What is wet stacking, and how do I know if my generator has it?

Wet stacking is unburned fuel and carbon residue accumulating in the engine from chronic under-loading — symptoms include black or oily exhaust residue, fouled injectors, and reduced performance. We cover the full mechanics, causes and fixes in our dedicated Generator Load Bank Testing guide.

Do I need a load bank test, and how often?

Annual is the figure most consistently cited — both as NFPA 110's fallback requirement when a building's real load can't exercise the generator properly, and independently as good practice from dealers and technicians regardless of formal compliance obligations. It's the most reliable way to confirm a generator can actually carry its full rated load when it matters, not just start.

Why is my generator's air filter clogging so fast?

Almost always the operating environment. Dusty, rural or open-shed conditions — common on Australian farms — load up air filters far faster than the manufacturer's baseline interval assumes. If your generator lives somewhere genuinely dusty, halving the typical service interval (down to roughly 250 hours) is a sensible adjustment rather than waiting for the standard interval to catch a problem.

Does a portable petrol generator need the same maintenance as a diesel standby unit?

Not quite — the failure modes differ. Petrol/small-engine units are more prone to carburettor gumming and ethanol-related water absorption in stale fuel. Diesel units don't have the ethanol problem, but they have their own idle-specific risks — microbial "diesel bug" contamination and wet stacking — that petrol engines generally don't experience in the same way. Don't assume maintenance advice transfers directly between the two.

What's the difference between a minor and major generator service?

It varies by manufacturer, but FG Wilson's terminology is a useful, directly citable example: a minor service at 500 hours (fluids, filters, inspections) and a major service at 1,000 hours (deeper component-level work). Check your own model's schedule for the equivalent split.

How do I know if my transfer switch will actually work when the power goes out?

Don't just watch it start the generator on a scheduled exercise cycle — that only proves the start signal works. Exercise it under real transferred load monthly as part of your generator run, and a couple of times a year, do a genuine "plugs out" test that disconnects from mains entirely and lets the switch do its actual job.

Are there Australian standards that require standby generator testing?

Not in the way NFPA 110 does in the US — there's no direct Australian equivalent mandating engine testing intervals. AS/NZS 3010 covers installation and electrical safety (including RCD testing obligations) rather than engine maintenance scheduling, and AS 1940 covers safe fuel storage rather than servicing. Some sectors have their own internal norms — poultry operations, for instance, are advised to test standby generators at least fortnightly — but for most standby diesel generator owners in Australia, following a documented schedule is a matter of good practice and reliability rather than a specific legal testing mandate.