Portable Generator Troubleshooting Guide: Step by Step

To use this portable generator troubleshooting guide, work through the checks in order: fuel, air, spark, then load. Most portable generators that will not start, stop soon after starting, run rough, or make no power have a fault in one of those areas. Generators run outdoors only, far from doors and windows, because of carbon monoxide, so do every check outside in open air.

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The quick answer: portable generator troubleshooting guide

Start with fuel: a generator that will not start often has an empty tank, a closed fuel valve, or stale fuel. Next check air and spark: a dirty air filter chokes the engine, and a fouled spark plug gives a weak spark or none at all. Then check oil, since many portable generators shut down when the oil level drops. Then check the load: a generator that stops or trips under load may be asked to run more than it can supply. Change only one thing at a time, so you know which change fixed it.

Symptom	First check
Will not start	Fuel valve, fuel level, choke, oil level
Starts, then stops	Stale fuel, air filter, oil level, vent
Runs but makes no power	Breaker, outlet, plug, load
Runs rough or surges	Stale fuel, air filter, spark plug, load
Shuts down under load	Total load, extension cord, breaker

Will not start at all

A generator that will not start at all usually lacks fuel, air, spark, or oil.

1. Fuel valve and fuel level: open the fuel valve and check that the tank holds fresh fuel. A valve left closed after storage is a common cause, and a tank that looks full can still hold stale fuel.
2. Choke: set the choke as the manual directs for a cold start, then open it as the engine warms. A choke left closed floods the engine.
3. Oil level: check the oil on a level surface and top it up if the level sits low. Many portable generators will not start when the oil level drops.
4. Spark plug: pull the plug and look for fouling or a cracked insulator. A plug that is wet with fuel needs drying, and a plug that is black with soot needs cleaning or a new plug.
5. Air filter: take the filter out and hold it to the light. A filter clogged with dust chokes the engine, so clean it or fit a new filter.
6. Fuel flow: if the tank holds fuel but none reaches the carburetor, the fuel line or filter may be blocked. Clear the line and the filter.

Starts, then stops

A generator that starts and then stops soon after usually runs out of fuel, air, or oil.

1. Stale fuel: fuel left in the tank over storage goes stale and gums up the carburetor. Drain the old fuel and refill with fresh fuel. See how to use fuel stabilizer.
2. Fuel cap vent: a blocked vent starves the engine as fuel leaves the tank. Loosen the cap; if the generator keeps running with the cap loose, clean or replace the cap.
3. Air filter: a dirty filter lets the engine start, then chokes it as the load rises. Clean or replace the filter.
4. Oil level: a low oil level trips the low-oil shutdown after a short run. Check the oil and top it up.
5. Load at startup: unplug every load and start the generator with nothing plugged in. If it keeps running, plug loads in one at a time.

Runs but makes no power

An engine that runs well but makes no power usually has a tripped breaker, a loose plug, or a load problem. See the how to fix generator not producing power guide.

1. Breaker and reset: find the breaker or reset button on the generator panel and reset it.
2. Outlet and plug: try a different outlet on the generator, and check that the plug seats firmly in the outlet. For plug shapes, see the types of generator plugs guide.
3. Load: unplug everything, then plug in a small load you know works, such as a lamp. If the lamp lights, the generator makes power.
4. Extension cord: a long or thin cord drops voltage and can trip the breaker. Use a shorter cord. See what size extension cord for generator.
5. Idle too long: a generator left idle for a long stretch can lose the magnetism that starts power production. If the outlets stay dead, a service shop can restore the field.

Runs rough or surges

A generator that surges or runs rough usually gets uneven fuel or air, or the load keeps changing.

1. Stale fuel: drain stale fuel and refill with fresh fuel, since stale fuel burns unevenly.
2. Air filter: clean or replace a dirty filter so the engine gets steady air.
3. Spark plug: clean or replace a fouled plug, since a weak spark misfires under load.
4. Load changes: loads that switch on and off, such as a fridge compressor or a pump, make the engine hunt. Spread the loads across the outlets.
5. Governor and carburetor: if fresh fuel, a clean filter, and a good plug do not steady the engine, the governor linkage or the carburetor may need cleaning. Leave that work to a service shop.

Shuts down under load

A generator that runs fine with no load but shuts down when loads plug in is usually overloaded.

1. Add up the load: list every load you plan to run and add their watts together, then compare the total with the watts the generator states it can supply.
2. Start big loads first: start the largest motor load first, such as a fridge or a pump, and let it settle before the next load.
3. Extension cord: a long cord adds load through voltage drop. Shorten the cord or move the generator closer to the loads.
4. Breaker: if the breaker trips at once when a load plugs in, that load may have a fault. Try a different outlet, and if it trips again, leave it unplugged.

Ways to keep it running well

• Run the generator with a load now and then through the year, so stale fuel does not build up.
• Store it with fresh fuel and stabilizer, or drain the tank before storage. See how to remove gas from a generator.
• Check the oil before each run and change it as the manual directs. See how often to change generator oil.
• Keep the air filter clean and the cooling vents clear of dust and debris.
• Cover the generator when it sits outside. For outage use, see how to use a generator during a power outage.
• Learn the safety basics before the power goes out: portable generator safety tips.

FAQ: power away from the grid

Why will my generator not start after storage?

Stale fuel is the usual cause. Fuel left in the tank over storage goes stale and gums up the carburetor, and a closed fuel valve or a low oil level stops it too. Drain the old fuel, refill with fresh fuel, and check the oil.

Why does my generator start and then stop?

A blocked fuel cap vent, stale fuel, a dirty air filter, or a low oil level are the usual causes. Loosen the cap, check the fuel and the filter, and check the oil.

Why does my generator run but make no power?

Check the breaker or reset button first, then the outlet and the plug, then the load. A tripped breaker, a loose plug, or a faulty load are the usual causes.

Can I run a generator indoors during an outage?

No. Never run a generator in a garage, a shed, or a basement, even with a door open. Keep it outside in open air, far from doors and windows.

How do I know if my generator is overloaded?

The breaker trips, the engine slows or stalls, or the generator shuts down when loads plug in. Add up the watts of every load and compare the total with the watts the generator states it can supply.

Related solar guides

See the how to use a portable generator, the gas generator maintenance guide, the inverter vs generator for home backup guide and the how does a portable generator work guide. For camping and emergency gear, see every Camping and Emergency Gear guide.