Putting the wrong fuel in your car is one of the most common mistakes drivers make at the pump. Every year, thousands of UK motorists accidentally fill a petrol vehicle with diesel or put petrol in a diesel car, often when they're distracted or driving an unfamiliar vehicle.
While using the wrong fuel can be alarming, acting quickly can often prevent expensive damage. The most important thing is not to start the engine if you've realised your mistake.
In this guide, we'll explain what happens if you put petrol in a diesel car, what happens if you put diesel in a petrol car, what you should do immediately after misfuelling, and how to avoid making the same mistake again.
What is misfuelling?
Misfuelling occurs when a driver puts the wrong type of fuel into their vehicle. Diesel vehicles are more prone to misfuelling as they have a wider fuel filler, which can easily take a petrol pump nozzle, whereas the neck of the petrol filler is tighter than a diesel pump nozzle, making it harder to use the wrong fuel.
The most common types of misfuelling are:
Putting petrol in a diesel car.
Putting diesel in a petrol car.
Filling the fuel tank with AdBlue.
Putting fuel into the AdBlue tank.
Of these, putting petrol into a diesel vehicle is generally considered the most serious, as modern diesel engines rely on diesel fuel to lubricate sensitive components in the fuel system.
What happens if you put petrol in a diesel car?
Putting petrol in a diesel car is generally more serious than putting diesel into a petrol vehicle.
Diesel acts as both a fuel and a lubricant for components such as the high-pressure fuel pump and injectors. Petrol removes this lubrication, increasing friction and potentially causing extensive damage if the engine is started.
If only a very small amount of petrol has been added to a nearly full diesel tank, damage may be minimal. However, modern common rail diesel engines are particularly sensitive, and manufacturers generally recommend draining the fuel system before driving.
If you've realised your mistake before turning the ignition on, do not start the engine. A fuel drain and system flush can often prevent further damage.
What happens if you put diesel in a petrol car?
Putting diesel in a petrol car is usually less damaging, but can still prevent the vehicle from running correctly.
Petrol engines rely on spark plugs to ignite fuel. Diesel is thicker than petrol and does not ignite in the same way, meaning it can clog the fuel system and prevent combustion.
Common symptoms include:
Difficulty starting.
The engine is refusing to start.
Excessive exhaust smoke.
Engine misfires.
Reduced performance.
If the engine hasn't been started, draining the contaminated fuel is normally enough to resolve the problem.
Can you drive after putting the wrong fuel in your car?
No. If you've accidentally put the wrong fuel in your car, you should avoid driving it.
Even if the engine appears to run normally, contaminated fuel can quickly circulate through the fuel system and increase the risk of costly repairs.
If you realise you've used the wrong fuel before starting the engine, leave the ignition off and arrange for the vehicle to be professionally drained.
If you've already driven the vehicle, stop as soon as it's safe to do so and contact your breakdown provider.
What is the cost of misfuelling?
The cost of misfuelling can vary significantly depending on your situation. If you immediately realise your mistake at the pump, then a mechanic should be able to flush the fuel system out without causing internal damage to your vehicle.
However, the damage is more severe if the ignition has been started as this could result in an expensive repair of your vehicle’s fuel system or you could need a complete replacement of the fuel pump, diesel injectors, filters, and fuel tank.
The overall cost depends on several factors, including:
The type of vehicle.
Whether the engine was started.
How much incorrect fuel was added.
Whether the vehicle was driven.
If caught early, draining and flushing the fuel system is often significantly cheaper than replacing damaged injectors, pumps or catalytic components.
How do you know if you’ve used the wrong fuel?
The symptoms depend on which type of fuel has been added. Some vehicles won't start at all, while others may begin showing problems shortly after driving away. However, if petrol has been used in a diesel vehicle, there could be a few warning signs which are:
A loud knocking sound as you accelerate.
Excessive smoke from the exhaust.
Slow acceleration.
Engine warning light is on.
The vehicle will not restart.
If you experience any of the above, then you should pull over when it is safe to do so and call a local garage or the AA, who’ll be able to help you.
What should you do if you’ve used the wrong fuel in your vehicle?
If you've realised you've put the wrong fuel in your car, staying calm can save you thousands of pounds. The biggest mistake is starting the engine, as this allows contaminated fuel to circulate through the fuel system.
Turning on the ignition after misfuelling causes the most damage to your vehicle, so it’s vital that you avoid doing this if possible. If you’ve realised you’ve used the wrong fuel before starting your engine, you should:
Leave the engine switched off and avoid putting your key in the ignition.
Inform the staff at the petrol station what’s happened.
Put the car into neutral.
If requested by the petrol station staff, push the vehicle to a safe place.
Call your breakdown company or a local garage to drain and flush your fuel system.
Call your insurance provider, as your policy may not cover misfuelling.
Report the incident to your fleet manager.
What should you do if you’ve started your engine after misfuelling?
Although misfuelling is common, there are several simple ways to reduce the risk:
Double-check the pump before filling up.
Take extra care when driving a hire car or company vehicle.
Avoid distractions while refuelling.
Add a reminder sticker inside the filler cap.
Consider fitting an anti-misfuelling device if you regularly drive diesel vehicles.
Businesses operating fleets can also reduce misfuelling incidents by providing driver training and using fuel cards that record vehicle fuel purchases, making it easier to identify unusual transactions.
How can you prevent misfuelling?
The best way to avoid misfuelling is to put a sticker on the filler cover to remind you what fuel is required.
There are also handy anti-misfuelling devices you can implement in your fleet vehicles, such as a misfuelling prevention cap which only permits the exact diameter of a standard diesel pump nozzle, preventing drivers from accidentally using petrol.
Understanding petrol, diesel and AdBlue
Knowing the difference between fuels can help prevent accidental misfuelling.
Petrol - Designed for spark ignition engines and available in standard unleaded and premium grades.
Diesel - Used by compression ignition engines and provides lubrication for key fuel system components.
AdBlue - AdBlue isn't a fuel. It's a diesel exhaust fluid used in many modern diesel vehicles to reduce harmful NOx emissions. It should only ever be added to the dedicated AdBlue tank.
Missfueling frequently asked questions
What should I do if I put the wrong fuel in my car?
Don't start the engine. Inform the petrol station if necessary, move the vehicle safely if instructed, and contact your breakdown provider or a fuel drain specialist.
Is putting petrol in a diesel car worse than putting diesel in a petrol car?
Yes. Petrol removes the lubrication that diesel engines rely on, increasing the risk of damage to pumps and injectors.
Will my insurance cover misfuelling?
Some comprehensive policies include accidental misfuelling, while others don't. Check your policy wording or contact your insurer.
Can a small amount of the wrong fuel damage my car?
It depends on the vehicle, fuel type and quantity added. Modern diesel engines are particularly sensitive to petrol contamination.
How can I prevent putting the wrong fuel in my car?
Double-check the pump label before filling up, avoid distractions, use reminder stickers, and consider an anti-misfuelling device for diesel vehicles.
