Putting the wrong fuel in your car is an embarrassing mistake, but it's much more common than you'd think. Whether you've put petrol in a diesel car or vice versa, do not start your vehicle – otherwise it can lead to costly damage.
Don't turn on the ignition or start the car – call us on 0370 240 3985
What you should do immediately
If you think you've put the wrong fuel in your car, make sure to do the following:
1. Do not start your car
- If you already have started your car, stop and switch off the ignition as soon as it's safe to do so.
2. Move your vehicle to a safe area
- Put the car in neutral.
- Get help to move your car to a quiet spot.
3. Call us
- Call our Fuel Assist team who can come out to drain the wrong fuel and replace it with enough correct fuel to get you going.
Why is using the correct fuel so important?
Diesel engines use diesel fuel for compression ignition in the engine, and it also lubricates the moving parts of the fuel pump system.
Petrol cars have an internal combustion engine, where the spark plugs ignite a petrol-air mixture to power the engine's pistons.
Unfortunately, putting the wrong fuel in your car can have serious consequences. It's worse to put petrol in a diesel car than it is to put diesel in a petrol car, but either way, you could sustain damage to your engine and its components. Incorrect fuel might ignite at the incorrect time, or cause clogs in the fuel system, leading to reduced performance, stalling, and complete breakdowns.
Effects of putting diesel in a petrol car
Accidentally putting diesel fuel in a petrol car isn't as bad as putting petrol fuel in a diesel car, but it can still lead to engine damage and other complications. The longer your engine is running with diesel fuel in it, the more damage will be caused – so, if possible, don't start the engine if you've accidentally put diesel in.
Here's what happens when you put diesel in a petrol car:
- Diesel fuel does not combust correctly in a petrol engine, meaning a lack of power for the car.
- The diesel fuel clogs up the spark plugs and the fuel system, leading to further reductions in performance and power.
- If you run your petrol engine with diesel fuel in it, it can create excessive pressure and heat, damaging the catalytic converter (potentially permanently).
It's important to take immediate action if you've put diesel in a petrol car, call our fuel assist team and they'll be able to come and flush out the incorrect fuel, and replace it with the correct type.
Effects of putting petrol in a diesel car
Petrol fuel is a less viscous, lighter fuel than diesel, and it's not compatible with diesel engines as it can't properly lubricate the fuel system. Rather, petrol fuel acts as a solvent and increases friction between the parts in a diesel engine.
Here's what happens when you put petrol in a diesel car:
- If there's some diesel fuel remaining in your tank and you introduce petrol, the two fuels won't mix well.
- The mixture of diesel and (mostly) petrol will be sent to the fuel filter and fuel injectors, but the fuel filter won't be able to separate the petrol away from the diesel.
- The petrol fuel will act as a solvent for the fuel injectors and pumps, dissolving the lubricating oils in the system and causing damage to the engine components.
- Petrol also won't combust properly in a diesel engine, which leads to increased wear and damage to the cylinders and piston rings
- The petrol will likely also damage the catalytic converter when it burns, as it creates excessive heat and pressure that the engine can't handle.
- If you've started your diesel engine with petrol fuel in it, the car will likely run erratically or stall completely.
Again, as soon as you realise you've put the wrong fuel in your car, you need to take immediate action and get a professional mechanic to drain and flush the fuel out of your car.
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What factors affect recovery and repair?
While any amount of wrong fuel in any type of engine is far from ideal, there are some factors that can affect the recovery and repair:
- The type of fuel – petrol in a diesel engine is more of a serious situation requiring extensive flushing and cleaning work. Diesel in a petrol engine might not cause any permanent damage (if you don't start your engine) but it still needs to be properly flushed.
- The amount of wrong fuel used – when it comes to incorrect fuel, the more you've accidentally added, the worse it is for the engine, leading to a longer repair time.
- The amount of time since you added the wrong fuel – if for example, you've left the wrong fuel in your car overnight, the recovery time could be much more time-consuming, as more damage could have been caused.
- Make and model/fuel tank capacity – the make can determine the complexity of the engine and fuel system and how easy it is to clean out, as well as the size of the fuel tank. The bigger the fuel tank, the more incorrect fuel it can hold, leading to greater damage.
- The skill of the mechanic carrying out the repair – an experienced mechanic will be able to efficiently diagnose the fault, flush out the incorrect fuel accordingly, and get you moving again.
- Whether or not you have breakdown cover – having breakdown cover can make recovery and repair a breeze. If you have breakdown cover with us you can use our app to get help. Don't worry though, even if you don't have cover, you can still get instant breakdown cover.
How to prevent misfuelling
There are a few ways we can make it less likely to fill up with the wrong fuel:
- Don't rely on the colour of the hose or nozzle before filling up – look for other clues.
- Properly read the pump's trigger label and the fuel grade indicator.
- Put reminders in the car, like a sticker inside the fuel cap.
- Distractions aren't worth it, so pop your phone down and give the pumps your full attention.
- Don't get fuel if you're in a hurry, hungry or stressed – you're more likely to make a mistake.
What if you put AdBlue in your diesel fuel tank?
It can be an easy mistake to put AdBlue in your fuel tank. The small blue filler cap is often next to the black diesel filler cap. Here's what to do:
- Don't turn on the ignition or start the engine – you're likely to face a big repair bill if you do.
- The AdBlue could cause expensive damage to your fuel tank, pump and injection system.
- Please call us on 0370 240 3985 and we'll help you.
We can't remove AdBlue by the side of the road because it's corrosive. That means we may need to tow you to a local garage to fix your vehicle.
What if you put diesel fuel in your AdBlue tank?
The AdBlue filler cap is smaller than the black diesel filler cap, so adding fuel here with a nozzle is less likely.
But if you do start your engine with fuel in your AdBlue tank, you might have to get the entire Selective Catalytic Reduction and AdBlue injection system replaced.
- Don't turn on the ignition or start the engine.
- Please call us on 0370 240 3985 and we'll help you.
Will 5 litres of petrol damage a diesel engine?
Yes, 5 litres of petrol in a diesel engine is more than enough to cause catastrophic damage if left long enough, and particularly if you start the engine.
Conclusion
Putting the wrong fuel in your car can be a costly mistake if you run the engine or if you put a lot of it into your car and leave it to sit. The most important thing to do is to call for help the second you realise you've used the wrong fuel. Our expert Fuel Assist team can help with any type of misfuelling issue, so make sure to give them a call and they'll do their best to get you back on the road with minimal damage.
Author: The AA. Updated 14 May 2025.