Standby generators fail to start for a small number of reasons, and they are heavily weighted toward one of them.
1. The battery — most of the time
A standby generator battery lasts three to five years, and it fails in a way that hides itself.
The weekly exercise cycle runs the engine for ten to twenty minutes, which recharges the battery and proves the engine works. What it does not prove is that the battery can deliver cranking current to a cold engine — and that is a completely different demand.
So a battery on its last legs passes every weekly test right up to the night it has to start a cold unit at 2am.
What to look for: the controller shows a low-battery warning, or the starter turns slowly, or it clicks and does nothing. A voltmeter across the terminals should read above 12.5 V at rest.
Replace it every three years on schedule, not when it fails. It is the cheapest thing on the whole machine and it is the failure most likely to happen.
2. The controller is in a fault it never cleared
Modern controllers lock out after a set number of failed start attempts and stay locked out. The unit then sits doing nothing, including skipping its exercise cycles.
If the display shows a fault, write the code down before clearing it — it is the single most useful thing you can hand a technician. Then reset and try again.
A unit that locks out again immediately has a real fault. A unit that runs fine afterwards had a transient one, and it is still worth knowing what it was.
3. Fuel — pressure, not quantity
Natural gas: the supply is there but the pressure may not be. A generator is a large appliance, and if the gas meter was sized for a furnace and a water heater, adding a 22 kW generator can exceed it — particularly on the coldest night, when the furnace is also at full rate. The symptom is an engine that starts and then stalls under load rather than one that will not crank.
Propane: two failure modes people do not expect. A tank below about 20% may not maintain pressure, and in very cold weather a tank cannot vaporise fast enough — a small tank on a large generator freezes its own regulator. Check the level and the tank size against the unit's draw.
Always check the shutoff valve. It gets closed during service and other gas work and not reopened.
4. Cold
Below freezing, oil thickens and batteries lose a large share of their cranking capacity — exactly when demand for the generator peaks.
Most standby units offer a cold weather kit: a battery warmer and an oil heater. In a northern climate it is not an accessory, it is the difference between a machine that starts in January and one that does not.
5. It was left in the wrong mode
The controller has OFF, AUTO and MANUAL. After any service it can be left in OFF or MANUAL, and in either the unit will not start on its own.
This is worth checking first because it takes five seconds, and it accounts for more no-starts than anyone admits.
The maintenance that actually prevents this
Annual service: oil, filter, spark plugs, air filter, and a visual check. Roughly $200–$500.
Battery every three years, on the calendar.
A load test at the annual service, not just an exercise. This is the important one. The weekly exercise usually runs without transferring, so it never tests the transfer switch and never tests the machine under real load. A generator can pass fifty-two exercises and still fail to power the house.
Keep the exercise cycle enabled. Some owners disable it because of the noise. That cycle is what charges the battery, circulates oil, and keeps fuel from going stale — turning it off is how a unit quietly dies between outages.
The honest expectation
A standby generator is a machine that sits still for months and is then asked to work perfectly on the worst night of the year. That only happens with maintenance on a schedule, and the schedule is short: battery, oil, load test.