Why won't my standby generator start?
Updated August 2026
A standby generator that cranks slowly or does nothing when the power fails is, in the large majority of cases, a dead or weak starter battery, which is the single most common reason these units refuse to start. The same lead-acid battery that starts a car sits idle for months in a generator, self-discharging until it can no longer turn the engine, and it needs replacing roughly every 3 years. If the battery is strong and it still will not fire, fuel, oil, and the spark plug are next.
Safety first: a standby generator burns fuel and produces carbon monoxide, so never troubleshoot or run one inside a garage or close to windows and vents, and never force a natural-gas or propane unit that smells of gas: shut the fuel valve and call the utility. Work on the battery and engine only with the unit set to OFF so it cannot auto-start and crank on your hands, and leave transfer-switch and wiring work to a licensed pro, because this equipment ties directly into your home's 240-volt service.
Causes, most likely first
| Cause | How likely | The fix | Typical cost |
|---|---|---|---|
| A dead or weak starter battery | Most common | Test or replace the battery, keep it on the unit's charger or a maintainer, and swap it about every 3 years. | $100–200 |
| Stale fuel or a fuel-supply problem | Common | On gasoline units, old fuel gums the carburetor; on propane or natural gas, check the supply valve and regulator. | $0–300 |
| A low-oil shutoff holding the engine off | Common | The safety sensor blocks starting when oil is low or the unit is not level; top up to the correct oil level. | $0–30 |
| The unit left in OFF or MANUAL, or a tripped generator breaker | Common | Confirm the controller is set to AUTO and reset the generator's own output breaker. | Free |
| A fouled or worn spark plug | Occasional | A carbon-fouled plug will not fire; clean or replace it and set the gap. | $10–30 |
| A controller or ignition-module fault | Occasional | A control-board error or bad ignition module blocks the auto-start; a technician diagnoses it. | $150–350 |
| A clogged carburetor or air filter | Occasional | A gummed carburetor or choked air filter on a gas unit prevents starting; clean or rebuild it. | $75–300 |
Check these first
- Check the controller display for an error code or a low-battery warning, and confirm the unit is set to AUTO, not OFF or MANUAL.
- Test the starter battery voltage or try a manual start; a slow, labored crank almost always means a tired battery.
- Confirm the fuel supply: propane tank level and valve, natural-gas service, or fresh gasoline with the shutoff open.
- Check the engine oil level, since a low-oil sensor will block starting entirely.
- Look at the generator's own output breaker and reset it if tripped, then attempt a manual start per the owner's manual.
- Review the exercise history; a unit that has not run in months is far more likely to have a flat battery and stale fuel.
Prevent it next time
Nearly every no-start traces to a battery or fuel that sat too long, and both are prevented by simply running the machine. Let it finish its weekly exercise cycle so the battery stays charged and the fuel stays fresh, change the oil on the recommended interval, and book the annual professional service that tests the battery, plug, and transfer switch. A generator kept this way starts when the grid drops; one that is ignored (see how long a whole-house generator lasts) is an expensive box that does nothing in the outage you bought it for.
When to call a pro
Handle the battery, oil, fuel valve, air filter, and spark plug yourself, but call a generator technician when the unit throws a controller fault, cranks but will not fire with good fuel and a fresh plug, or will not transfer the load when the grid drops. An annual service runs $150–350, a new starter battery $100–200, and a carburetor rebuild or control-board repair $200–600. Anything involving the automatic transfer switch or the 240-volt connection to your panel is licensed-electrician work, not DIY.
Most of the causes above are maintenance problems wearing a disguise. OnOtto keeps the boring prevention on schedule so the 2 a.m. version doesn't happen.
Stop chasing symptoms, free for 30 daysFrequently asked questions
How often should I replace my standby generator's battery?
Plan on a new battery about every 3 years, and sooner in very hot or very cold climates that shorten battery life. The battery is the number-one reason standby generators fail to start, because it sits mostly idle and self-discharges between the brief exercise runs. Keeping it on the unit's built-in charger or a trickle maintainer stretches its life, but even a well-kept battery weakens with age. Testing it during the annual service and replacing it on a three-year schedule, before it fails, is far cheaper than losing the generator mid-outage.
Why does my generator crank but not start?
A crank with no start means the battery is fine and the problem is fuel, spark, or a safety shutoff. On gasoline units, stale fuel and a gummed carburetor are the usual suspects after months of sitting. On propane and natural-gas units, check that the supply valve is open and the regulator is working. Then look at the spark plug, which can foul and stop firing, and the oil level, since a low-oil sensor blocks starting. If fresh fuel, a clean plug, and correct oil do not fix it, the controller or ignition needs a technician.
Do I really need to run my standby generator every week?
The weekly self-test is not optional if you want it to work. That short run, usually 5–20 minutes, recharges the battery, circulates oil, keeps seals and the carburetor from drying out, and burns off condensation in the engine. Skipping it is exactly how batteries go flat and fuel goes stale between outages. Most units run the exercise automatically on a schedule you set, so confirm it is enabled and that you can actually hear it run. A generator that never exercises is the one that stays silent when the grid fails.