If you've just done it
- Don't start the engine. Don't turn the ignition on either.
- Tell the forecourt staff.
- Put the car in neutral and push it somewhere safe, with help.
- Call your breakdown provider or a fuel drain service.
- Let your insurer know.
If you're already driving and the car starts to misbehave, pull over safely, switch off and call for help.
Misfuelling is an easy mistake. The RAC says it happens around once every three minutes in the UK, and Which?, citing RAC figures, put it at around 150,000 drivers a year in July 2024.
If you catch it before the engine runs, the fix is usually a fuel drain, not a repair.
If you notice before starting the engine
Stop there. According to the RAC, "the most serious damage from misfuelling occurs after turning on the ignition", because switching it on starts to circulate the mixture through the fuel system. So leave the key out of the ignition, or don't press the start button.
Then:
- tell the forecourt staff what's happened
- put the car in neutral and push it to a safe spot, with someone helping
- call your breakdown provider, or a specialist fuel drain service
A technician will drain the wrong fuel and put in enough of the right fuel to get you going. The AA's Fuel Assist service, for example, is open to non-members and says the drain itself takes around 45 minutes once the technician arrives.
Petrol in a diesel car: the more serious case
The RAC says putting petrol in a diesel car "is far more serious" than the other way round.
Diesel helps lubricate the parts of the fuel system. Petrol acts like a solvent and strips that lubrication away, which increases friction and can damage the fuel pump and fuel lines. The AA adds that it can damage injectors, wear cylinders and piston rings, and harm the catalytic converter. It also warns that 5 litres of petrol is enough to cause serious damage if the engine is started.
Diesel in a petrol car: usually less serious
This is less common, partly because, as Motorpoint explains, most diesel nozzles are too wide for a petrol filler neck. If it does happen, the RAC says diesel coats the spark plugs and fuel system, which usually causes misfiring, smoke or stalling. It still needs draining as soon as possible, and the AA notes it can damage the catalytic converter.
If you've already driven off
Signs of petrol in a diesel include knocking when you accelerate, smoke, poor acceleration, a warning light, stalling or trouble restarting. Diesel in a petrol car tends to cause misfiring, smoke and cutting out (RAC).
If you notice any of these after filling up:
- Pull over as soon as it's safe and legal to do so.
- Switch the engine off.
- Put the car in neutral and call your breakdown provider or a fuel drain service.
- Tell your insurer.
Don't drive on to a garage: the longer the engine runs, the further the wrong fuel spreads.
What it might cost
Prices vary with the car, the size of the tank and how far the fuel has travelled. In July 2024, Which? reported that draining the fuel costs between £120 and £1,000 in the best cases. It quoted the RAC as saying costs could reach up to £5,000 in extreme cases, where parts of the fuel system need replacing.
Ask for the price before you book. The AA says it quotes first, and its personal members can save up to 10%.
Will insurance or breakdown cover pay?
Not always. When Which? looked at 61 car insurance policies in 2024, 43% included misfuelling as standard and 10% offered it as an optional extra. Of 44 breakdown policies, 41% included misfuelling help as standard and 18% as an extra.
Where cover exists, it may have limits. Compare the Market says some policies won't pay for engine damage, or for damage caused by starting the car or knowingly driving off after misfuelling. You'll usually also pay your excess.
Check your policy documents before you need them.
How to avoid doing it again
- Read the label, not just the colour. Green handles for petrol and black for diesel have been the usual UK convention for years (Fleet News reported it in 2003), but the AA says not to rely on the colour of the hose or nozzle.
- Check the fuel code. Petrol pumps and nozzles carry a circular E10 or E5 label (gov.uk). Diesel is marked B7 in a square (Aviva). Newer cars often have a matching label inside the filler flap.
- Take extra care in an unfamiliar car. Hire cars, courtesy cars and a new car that uses a different fuel from your old one are the classic traps. Aviva suggests a reminder sticker in the car or on the fuel cap.
- Consider a misfuel prevention cap. These fit in the filler neck of a diesel car and block a petrol nozzle. Aviva says a diesel-specific cap can cost less than £30.
- Slow down. The AA suggests putting your phone away and not refuelling when you're rushed, hungry or stressed.
Questions
Can I drive with a little diesel in my petrol car?
It's not worth the risk. Even a small amount can cause misfiring and smoke, and the RAC advises draining the fuel system as soon as possible.
How much does it cost to fix putting petrol in a diesel?
If you haven't started the engine, it's usually just a fuel drain: Which? reported £120 to £1,000 in 2024. If the engine has run, repairs can cost much more, with the RAC quoting up to £5,000 in extreme cases.
Does insurance cover putting the wrong fuel in?
Sometimes. Less than half of the car insurance policies Which? reviewed in 2024 included it as standard, and some exclude engine damage. Check your policy.
I only turned the key and didn't start it. Is that a problem?
Possibly. The RAC says the worst damage starts once the ignition is on, so treat it as if the engine may have circulated some fuel, and still call for a drain.
Once you're back on the road, you can search by postcode to find the cheapest E10 petrol and B7 diesel near you. Not sure which petrol grade your car needs? Our guide to E10 vs E5 petrol explains the difference.