You flip the switch or pull the cord and nothing happens, usually on the night you need backup power most. The good news: a generator that cranks but won’t fire (or won’t crank at all) almost always has a simple, fixable cause. The large majority of no-start problems come down to fuel, oil, or the choke, and you can check all three in a few minutes with no special tools. Work through the steps below in order before you assume the worst.
⚠️ Troubleshoot outdoors, never add fuel to a hot engine
A portable generator gives off carbon monoxide, an invisible, odorless gas that can kill within minutes. Only run or test one outdoors, at least 20 feet from the house with the exhaust pointed away from windows, doors, and vents — never in a garage, shed, or basement, even with the door open (CPSC). Before you add gas, turn the engine off and let it cool; fuel spilled on a hot engine can ignite. Follow your owner’s manual, and leave fuel-system or electrical repairs to a qualified technician.
Check the fuel first
Fuel is the single most common reason a generator won’t start, so begin here. Confirm the tank actually has gasoline in it — gauges stick, and a unit that “ran fine last time” may simply be empty. Then check two things most people miss:
- The fuel valve (petcock). Make sure it is turned to the open or “on” position. If it was left closed after the last run, no fuel reaches the carburetor no matter how many times you pull.
- The age of the gas. Gasoline degrades surprisingly fast. Manufacturers warn that ordinary pump fuel can start to break down in as little as a few weeks to a month, leaving gum and varnish that clog the carburetor. If the gas has been sitting since last season, drain it and refill with fresh fuel (ideally treated with stabilizer).
If you keep running into stale-fuel problems, it helps to know roughly how long a tank lasts so you can size your fuel storage and rotate it. Our guide to generator fuel use breaks that down by load. And whenever you do refuel, follow the same outdoor, engine-off rules covered in using a generator safely.
Check the oil level
Nearly every modern portable generator has a low-oil shutoff sensor. If the oil is below a safe level, the sensor cuts the ignition so the engine won’t start — a feature designed to protect the engine, not a fault. This catches a lot of people off guard, especially on a generator parked on a slope, where the oil pools away from the sensor.
Put the generator on level ground and check the dipstick (most should read full, with the cap resting on the threads rather than screwed in — check your manual). Top up with the grade of oil your manual specifies if it is low. If the oil looks correct and full but the unit still won’t start, a failing low-oil sensor can falsely block ignition; that is a diagnosis better confirmed by a technician than guessed at.
Set the choke and switches
A cold engine needs a richer fuel mixture to fire, which is what the choke provides. Two quick checks here clear up a lot of “it just won’t catch” complaints:
- Choke position. For a cold start, move the choke to the closed or “start” position. Once the engine fires and runs for a few seconds, move it back to open or “run.” Starting a warm engine with the choke fully closed can flood it instead.
- Engine and fuel switches. Confirm the engine on/off switch is set to “on,” and that any separate fuel switch is open. It sounds obvious, but a single switch left in the wrong position is a common culprit.
- Remove the load before starting. Unplug everything before you start the engine. Manufacturers advise that nothing should be plugged in during startup — even an empty extension cord — because a connected load makes the engine harder to spin up.
Spark plug and air filter
If fuel, oil, and the choke all check out, move on to spark and air. The engine needs a clean spark and a clear path for air to ignite the fuel.
- Spark plug. Remove the plug with the wrench supplied with the generator and inspect it. Black, oily, or sooty deposits (fouling), cracks, or a wet plug all point to a problem. Clean light deposits, check the gap against your manual’s spec, and replace the plug if it is worn or damaged — they are inexpensive.
- Air filter. A filter clogged with dust or soaked in oil starves the engine of air and can prevent starting. Pull the element and clean or replace it if it is dirty. Never run the engine with the filter removed.
Carburetor and stale fuel
When a generator has sat for months, the most likely hidden cause is a clogged carburetor. As gasoline evaporates it leaves behind sticky residue that blocks the tiny jets and orifices the engine relies on. The symptom is classic: plenty of fuel in the tank, good spark, but the engine won’t fire or only runs while you spray starting fluid.
Draining the old fuel and cleaning the carburetor bowl and jets often brings these units back to life. This is a job some owners are comfortable doing with carburetor cleaner and basic tools, but it involves disassembling the fuel system — if you are not confident, it is a good point to stop and call a pro. If recurring stale-gas headaches are wearing you down, it is one reason some buyers prefer propane, which doesn’t gum up the same way; our dual-fuel vs gas comparison weighs that trade-off.
Electric-start battery
If your generator has an electric starter and the engine won’t even crank — you hear a click or nothing at all when you turn the key — the starter battery is the first suspect. These small batteries lose charge while the unit sits in storage.
Check that the battery is charged and that the terminals are clean and tight. Most electric-start models also have a recoil (pull) cord as a backup, so you can usually start the unit by hand to confirm the rest of the engine is healthy while you charge or replace the battery. If the engine cranks fine on the pull cord but not on the key, the issue is in the battery or starter circuit rather than the engine itself.
When to call a pro
Most of the checks above are owner-level maintenance. Stop and bring in a qualified small-engine technician (or use your warranty service) when:
- You have confirmed fuel, oil, choke, spark, and air are all correct and it still won’t start.
- The carburetor needs to come apart and you aren’t comfortable with fuel-system work.
- You suspect the low-oil sensor, ignition coil, starter, or wiring — electrical and ignition repairs.
- You smell a fuel leak, see fuel pooling, or the unit smoked or sparked.
- The generator is under warranty; DIY repairs can void coverage.
There is no shame in handing off a stubborn fuel or electrical problem — those repairs carry fire and shock risk, and a technician will be faster and safer than trial and error.
Quick troubleshooting table
| Symptom | Likely cause | Fix |
|---|---|---|
| Cranks but won’t fire, gas in tank | Fuel valve closed or stale gas / clogged carburetor | Open the fuel valve; drain old gas and refill fresh; clean the carburetor if needed |
| Won’t start after sitting for months | Gum and varnish from old fuel blocking the carburetor jets | Drain the tank, refill with fresh stabilized fuel, clean the carburetor (or call a pro) |
| Tries to start then dies immediately | Low oil triggering the shutoff sensor, or choke left closed | Level the unit and top up oil; move the choke to “run” once it fires |
| No spark / engine floods | Fouled, wet, or worn spark plug | Inspect the plug, clean or replace it, check the gap to spec |
| Sluggish or no start, weak power | Dirty or oil-soaked air filter | Clean or replace the air filter element |
| Click or no crank (electric start) | Dead starter battery | Charge or replace the battery; use the recoil pull cord as backup |
| Hard to start with cords attached | Electrical load connected during startup | Unplug everything, start the engine, then add the load |
Plan around what you actually need to power
Once your generator is running again, make sure it is matched to the job. Overloading a unit causes its own headaches; undersizing leaves you in the dark. Use the Power-Station Sizing calculator to figure out the wattage you need for your essentials, then check the Appliance Runtime calculator to see how long a tank or battery will keep them going.
Frequently asked questions
Why won’t my generator start after sitting all winter?
The usual cause is stale fuel. Gasoline left in the tank and carburetor over the off-season breaks down and leaves a gummy residue that clogs the carburetor’s small passages. Drain the old gas, refill with fresh fuel, and clean the carburetor if it still won’t fire. Going forward, run the unit dry or add fuel stabilizer before storage.
Can low oil stop a generator from starting?
Yes. Nearly all portable generators have a low-oil shutoff that prevents the engine from starting (or shuts it down) when oil is below a safe level, to protect the engine. Set the generator on level ground, check the dipstick, and top up with the specified oil. A unit parked on a slope can trip this sensor even when there is enough oil overall.
Where should the choke be when I start it?
For a cold engine, set the choke to closed or “start.” Once the engine catches and runs for a few seconds, move it to open or “run.” If the engine is already warm, it usually starts with little or no choke; leaving it fully closed on a warm engine can flood it.
Is it safe to start a generator in the garage to troubleshoot?
No. A portable generator produces carbon monoxide, which can build to deadly levels indoors within minutes — and an open garage door does not make it safe. The CPSC advises running a generator only outdoors, at least 20 feet from the home with the exhaust directed away from windows, doors, and vents. Do all starting and testing outside.
Should anything be plugged in when I start it?
No. Remove all electrical loads before starting. Manufacturers recommend that nothing be plugged into the generator during startup, including empty extension cords, because a connected load makes the engine harder to start. Start the engine first, let it stabilize, then plug in your devices.
Sources
- U.S. Consumer Product Safety Commission — Carbon Monoxide Information Center: https://www.cpsc.gov/Safety-Education/Safety-Education-Centers/Carbon-Monoxide-Information-Center
- U.S. Consumer Product Safety Commission — What to Know About Portable Generators and Carbon Monoxide (PDF): https://www.cpsc.gov/s3fs-public/468-WhattoKnowGenerators_2022.pdf
- FIRMAN Power Equipment — My Generator Won’t Start: Troubleshooting Steps: https://firmanpowerequipment.com/blogs/news/my-generator-wont-start-troubleshooting-steps
- WEN Products — 10 Common Reasons Your Generator Won’t Start: https://wenproducts.com/blogs/resource-center/how-to-start-my-generator