Misfuelling: what to do if you put the wrong fuel in your car

Filling your car up at a petrol station with the wrong fuel is easily done. So easily, in fact, that around 150,000 of us do it each year, according to the RAC.

Misfuelling can be disastrous if you're not careful, but if you realise your mistake in time, you can avoid any serious damage. We'll explain what to do if you accidentally put the wrong fuel in your car, including what you can claim from your insurance provider.

In This Guide:

How does misfuelling affect your car?
What to do if you put the wrong fuel in your car
Preventing misfuelling
Is damage from misfuelling covered by my car insurance?

How does misfuelling affect your car?

Petrol and diesel engines are more different than you might think, and so what actually happens when you put the wrong fuel in will depend on what type of engine your car has.

What happens when you put petrol in a diesel car

Putting petrol in a car with a diesel engine is more likely to cause damage than the other way round. Metal parts of the engine rely on the diesel fuel to lubricate them so they work properly. Petrol, however, acts as a solvent, prevents diesel's lubricating function, and causes the metal components to rub against each other.

This can cause debris to clog up the fuel injector. The petrol itself can also cause damage to the fuel lines, and if it gets into the fuel injectors, they will likely need replacing. This tends to come at vast expense.

What happens when you put diesel in a petrol car

Putting diesel in a petrol car isn't quite as bad. In most cases it won't cause significant lasting damage, but is likely to cause your car to misfire if you try and start it as the diesel coats key components like the spark plugs. Your car won't start, and might start kicking out nasty smoke, but should be fine once the fuel tank has been drained.

What to do if you put the wrong fuel in your car

It's perhaps best to start with what not to do: do not start your car.

If you realise as the pump that you've put the wrong fuel in, here's what you should do:

Let an attendant know what's happened
Put your car in neutral and push it somewhere out of the way
If you have breakdown cover, call your provider and have them come to drain and flush your fuel tank
Call your insurance company and inform them of what's happened

If you have already started your car, pull over as soon as you can safely and switch the engine off, the proceed from step 2 above.

Be warned that if it's a diesel car you've misfuelled, and you have started to drive off, the chances are high that the damage will be significant and costly to repair.

Preventing misfuelling

Aside from the obvious (always concentrate when you fill up your car), there are a couple of tips you can follow to make sure you don't put the wrong type of fuel in:

Label your fuel cap with petrol or diesel. This can be especially helpful if you've switched from one fuel type to another when buying a new car
Install a misfuelling prevention device. These devices slot into your car's filler neck and only allow the right sized nozzle to fit in (petrol nozzles are thinner than diesel nozzles)

Is damage from misfuelling covered by my car insurance?

It depends. Some insurers cover issues caused by misfuelling under accidental damage clauses, some don't. Check your policy documentation to be sure - if misfuelling isn't covered, it will normally say so explicitly. If you're still unsure, contact your insurance company - if you get a cheaper car insurance policy, it might not include a clause on this.

Most of the time, misfuelling isn't covered as standard. However, you may find that you can take out specific misfuelling cover as an optional extra. Even if you can this cover, it will normally only mean you get a pay out to cover the drainage and flushing of your fuel tank, but not any damage that occurs as a result of you driving off.