Why Your Generator Won’t Start (& How to Fix It)

April 24, 2026 | Nate Cord

Have a generator that refuses to start when you need it most? The good news is that most starting problems have simple causes that you can diagnose and often fix yourself.

In this comprehensive guide, our experts will walk you through the most common reasons a generator won’t start, with separate sections for petrol and diesel models. We’ll then help you work through each problem, from simple checks to more advanced fixes, including when it’s best to consult a specialist.

Before You Begin Troubleshooting

Before you start to poke around your generator, there are some important rules to follow to keep you safe. These are:

• Turn the engine off, then remove the key (start-up). If you’re inspecting components, you need to make sure the engine cannot accidentally start.
• Always work in a well-ventilated area. Never run or troubleshoot a generator indoors or in an enclosed space.
• Ensure the generator is on a level surface. This prevents fuel leaks and ensures accurate oil readings.
• Disconnect the spark plug wire. For petrol generators, this prevents accidental starting while you’re at work.
• Never smoke or allow naked flames near the generator. Fuel vapour is highly flammable and can cause a serious fire and/or explosion if not handled with care.

If you’re ever unsure about a procedure or if the problem seems beyond a simple fix, don’t hesitate to contact a professional.

What to Check When Your Generator Won’t Start

Before we dive into fuel-specific issues, we’ll first cover common reasons your generator won’t start that can be checked across all generator types. We’ll then look at petrol and diesel-specific issues if those initial checks don’t resolve your problem.

All Generators

Before we dive into fuel-specific issues, there are several checks that can be performed on all generators. Work through these first, as they’re the most common causes of starting failure.

1. Check the Fuel Level

It sounds obvious, but you’d be surprised how often this is the culprit. Check that there’s fuel in the tank. If it’s low, fill it up and try again.

To avoid further issues, do not use old or stale fuel. If the fuel is old (over two months), replace it with fresh fuel.

2. Check the Engine Oil Level

Generator oil is crucial for lubricating, cooling, and sealing engine components. Most modern generators have a low-oil shutdown sensor. If the oil level is too low, the engine won’t start; if it does start, it will quickly cut out. This is a safety feature to prevent engine damage.

Check the oil level by visually checking the level on the metal thread. If it’s low, top it up to the correct level with the recommended oil for your generator. Or you may be due a service so replace the oil if the service schedule states to.

Bear in mind that attempting to run the generator on an uneven surface might cause the low-oil sensor to trigger even when there is enough oil.

3. Check the Fuel Valve

Lots of generators have a fuel valve that must be in the “ON” or “OPEN” position for fuel to flow to the carburettor or injectors. Check that you haven’t left it in the “OFF” position after the last use.

If the valve is set to “OPEN” but the fuel still does not flow, first check for a vacuum relief valve on top of the tank or on the fuel cap and open it. Second, unplug the hose from the fuel valve’s intake side to see if it’s flowing (have a jerry can or a drip tray ready here). This will help you determine which part of the fuel line is clogged, if any.

4. Check the Choke Setting

If your generator has a manual choke, make sure it’s set correctly for it to start. For a cold engine, the choke should be in the “OPEN” or “CHOKE” position. Once the engine starts, move it to the “RUN” position.

Depending on the style of your generator, this choke lever may be positioned in several places.

If the engine is warm (i.e., if it has been running for a while and you are refuelling), you may not need the choke at all. Over-choking a warm engine can flood it, making it difficult to start. In this case, the choke should be set to “CLOSED” (“RUN”).

5. Check the Battery

If your generator has an electric start and is turning over slowly or not at all, the battery is often the culprit.

Ask yourself: Are the battery terminals clean and tight? Is the battery fully charged? Check the voltage level? Is the battery old? Batteries typically last 3-4 years. If yours is older, it might need to be replaced.

For generators with a recoil starter (pull cord), this isn’t an issue, but check that the cord doesn’t feel jammed.

6. Check the Circuit Breaker

If the generator’s main circuit breaker has tripped, it may prevent the engine from starting or cut power. Check the breaker and reset it if required.

If it’s popped out or won’t press back in, there’s likely still a fault somewhere in the generator.

7. Check for Blocked Air Filters

A clogged air filter can starve the engine of air, making it difficult or impossible to start. Remove the air filter cover and inspect the element. If it’s dirty, clean it or replace it.

8. Check the Spark Plug Connection

For petrol generators, ensure the spark plug wire is firmly connected to the spark plug. It can work loose over time due to vibration.

Over time, deposits and buildup can also appear on the spark. Clean the spark plug using a small tool, e.g., a wire brush, as needed.

Corrosion can also occur on the spark plug’s terminals and threads, especially in humid or wet environments. This means the spark plug will need to be replaced.

Petrol

If you’ve worked through the general checks and your petrol generator still won’t start, here are the next things to check for.

Spark Plug

A faulty or worn spark plug is a common culprit. The spark plug provides the ignition that starts the engine. In other words, if it’s not working, the engine won’t fire.

Remove the spark plug and inspect it. Is it wet with fuel? That suggests flooding. Is it covered in black soot? That suggests it’s worn or the engine is running too rich.

You should also check the electrode gap. Use a feeler gauge to check and, if needed, adjust.

What to do: Clean the spark plug with a wire brush if it’s just got dirt on it. If it’s worn, cracked or otherwise ruined, replace it.

Flooded Engine

If you’ve been trying to start the generator with the choke on for too long, the engine might flood. You’ll often be able to smell petrol if this is the case.

What to do: Turn the fuel valve OFF. With the choke in the RUN position, pull the starter cord repeatedly (or crank the engine) for 10-15 seconds to clear the excess fuel.

Clogged Fuel Filter

A blocked fuel filter can restrict fuel flow to the carburettor. The filter is usually located in the fuel line.

What to do: Consult your manual to locate the fuel filter. If it’s clogged, replace it. On some models, you can clean it, but replacement is usually the better option.

Low-Oil Sensor

Sometimes the low-oil sensor prevents the engine from starting. To prevent sensor malfunctions, ensure your generator is positioned on a stable, even surface before checking the oil level.

What to do: If you’ve confirmed the oil level is correct and the sensor appears to be preventing the engine from starting, you may need to have a dealer test and replace the sensor.

Carburettor Issues

If you’ve checked everything else and the generator still won’t start, the carburettor may be clogged or need adjustment. This is particularly common after long-term storage with old fuel.

To avoid this in the future, try to run your generator at least once a month, and never store it for long periods of time without draining the petrol tank and the carburettor first.

What to do: Carburettor cleaning and adjustment are jobs for a qualified technician. If a clean or adjustment is needed, you must contact an authorised mechanic/dealer.

Diesel

Diesel generators have different starting requirements than petrol models. Here’s what to check if your diesel won’t start.

Emergency Shut-Off Switch

Some diesel generators are fitted with an emergency shut-off switch or button. If this is accidentally pressed, it can stop the generator from starting.

What to do: Check that the emergency shut-off button or switch has been fully released and is not engaged.

Air in the Fuel System

This is one of the most common issues with diesel generators, after running out of fuel or after filter changes. Air in the fuel lines prevents the injectors from working as they should.

What to do: The fuel system needs to be bled to remove air. This procedure varies by model, so consult your manual. If you’re not confident, a mechanic can do this for you.

Glow Plug Issues

Some Diesel engines use glow plugs to heat the combustion chamber during cold starts. If the glow plugs aren’t working, the engine may struggle to start in cold weather.

Ask yourself: Does your generator have a pre-heat button or an automatic glow plug system? If so, ensure you’re using it as expected. On some models, for example, there’s a pre-heater button on the control panel.

What to do: If glow plugs are not working as expected, they’ll need to be tested and replaced.

Fuel Contamination

Diesel is susceptible to water contamination and microbial growth (diesel bug). Water in the fuel can prevent starting and cause serious damage to the injection system.

What to do: If you suspect the fuel is contaminated, drain the tank and replace the fuel filter. This job may require professional help, especially if there’s significant contamination.

Clogged Fuel Filter

Diesel fuel filters need regular replacement. A clogged filter restricts fuel flow and can prevent the engine from starting.

What to do: Replace the fuel filter at the intervals specified in your manual. This tends to be every 100-250 hours, depending on the model.

Fuel Shut-Off Solenoid Issues

Some diesel generators have a fuel-shutoff solenoid that controls fuel flow during starting and stopping. If this solenoid fails, fuel won’t reach the injectors.

What to do: This is a job for a professional. They can test the solenoid and replace it if needed.

Low Compression

If the engine turns over freely but won’t fire, and you’ve ruled out fuel and air issues, there may be a compression problem. This is serious and requires professional attention.

What to do: Contact a dealer for compression testing and diagnosis.

When to Call a Professional

While many starting problems have simple solutions, some issues require professional expertise.

• You’ve worked through all the checks, and the generator still won’t start.
• You’re not confident in your ability to perform the checks or fixes yourself.
• The generator is still under warranty (DIY repairs may void it)
• You suspect internal engine damage.
• The generator makes unusual knocking or grinding noises.
• There’s smoke from areas other than the exhaust.

The manuals are clear on this: If you are unsure of how to perform a maintenance task, have the unit serviced by a dealer. This applies to troubleshooting, too.

Conclusion

A generator that won’t start is almost always trying to tell you something. By working through these checks, you can often identify the problem and sometimes fix it yourself.

The best way to deal with a generator that won’t start is to prevent the problem in the first place. Regular generator maintenance dramatically reduces the risk of starting failures.

Do you have another question or require additional advice? Don’t hesitate to contact our experts via this form or via phone at +441639849847.

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