Wrong Fuel in Your Car? What to Do Right Now

Haven't started the engine? Don't

Leave the engine off. Don't turn the ignition on, and don't put the key in. Tell the forecourt staff what's happened, then call your breakdown provider.

That's the whole answer for the first minute. The RAC, the AA and the parts maker Delphi all say the same thing. The most serious damage comes once the ignition goes on, because that's what pumps the wrong fuel through the system. A tank full of the wrong stuff, sitting still, is a drain-and-flush job.

How bad the rest gets depends on which way round you did it. Petrol in a diesel is the expensive one. Diesel in a petrol is usually a simpler fix.

Part of our guide to running a car.

What to do, in order

First, tell someone. Let the staff at the filling station know, because you're about to block a pump for a while and they'll have seen this before.

Next, get the car out of the way without starting it. The RAC's advice is to put the car in neutral and push it to a safe place. If you can't push it, or the car won't go into neutral with the engine off, don't force it. Ask the staff or wait for the recovery crew and let them move it.

Then call your breakdown provider. If you haven't got cover, a mobile fuel-drain service or a local garage can do the same job. Tell them which fuel went in and roughly how much.

Keep the receipt from the pump, too. It's the one piece of evidence that shows what was sold to you. In one ombudsman case below, the driver not having it was one of the gaps in his evidence.

What happens next is a drain and flush. The crew empties the tank and fuel lines of the contaminated fuel, flushes the system, and puts in enough of the right fuel to get you moving again. The RAC says that can take anywhere from a few minutes to a few hours, depending on how much fuel went in. It also notes that modern cars have sealed tanks, which makes the wrong fuel harder to get at.

The AA says it can't remove AdBlue at the roadside. AdBlue is an exhaust fluid, not a fuel, and the AA says it's corrosive, so a car with AdBlue in the tank may need towing to a garage.

Petrol in a diesel, and diesel in a petrol

These are two different problems. People lump them together because the first move is the same, but the damage isn't.

Petrol in a diesel is the one to worry about. A diesel engine uses its fuel as a lubricant for the pump and injectors. Petrol does the opposite. The RAC says that when petrol is added to diesel, the mixture acts like a solvent and dissolves the lubricant.

Delphi, which makes fuel systems, puts it in numbers. Modern common-rail diesels inject fuel at pressures up to 2,000 bar, with injector tolerances measured in microns.

Delphi's view is that "even a small amount of petrol can cause major damage to the pump and injectors". It adds that the damage can spread quickly to the rest of the engine. The result could be a new pump, filter, injectors, pipes and rails, or even a whole new engine, "with a possible repair bill running into the thousands".

It's also the commoner mistake. A petrol nozzle is smaller than a diesel one, so it goes into a diesel filler without a fight.

Some diesels have a device that's meant to stop the wrong fuel going in. One ombudsman decision looked at a car with one fitted. The driver's expert said it was still possible to misfuel it, and it seemed to be agreed that doing so would mean a very slow fill. So a misfuelling guard helps, but it isn't a guarantee.

Diesel in a petrol car is the other way round. Diesel doesn't combust properly in a petrol engine.

If the engine is started, the AA and RAC say the diesel coats the spark plugs and fuel system. The car may misfire, smoke, lose power, cut out or fail to start. The AA says it may also damage the catalytic converter.

Delphi calls it a clogged fuel system and spark plugs, and says it's easily repairable. None of that is harmless. It's just that a flush usually puts it right, where a petrol-in-diesel can need parts.

People always ask how much is too much. There's no figure that everyone agrees is safe.

The RAC's FAQ says that more than 3% petrol in a diesel may bring poor running, stalling and possibly engine damage. Delphi says even a small amount can do major damage. So treat any amount as a drain job and let the recovery crew decide.

If you've started it or driven off

Stop as soon as it's safe, and switch the engine off. That's the AA's advice and the RAC's. The sooner the wrong fuel stops circulating, the less chance it has to do damage.

Don't be fooled by the car feeling fine at first. Haynes' technical editor Martynn Randall explains that the engine will likely run for a short while on the fuel already in the pipes. The trouble starts when that runs out and the contaminated fuel reaches the engine. So a car that pulled away cleanly can start coughing once you're down the road.

Nobody can tell you how far is too far. The AA, RAC and Delphi all say damage may already be done once the engine's run. None of them gives a distance, and I wouldn't trust any page that does.

If it dies on an ordinary road, the Highway Code says to try to stop in a place of relative safety, such as a lay-by. Get the car off the road if you can, put the hazard lights on if it's causing an obstruction, and keep away from moving traffic. A warning triangle goes at least 45 metres behind the car, and never on a motorway.

On a motorway, rule 277 says to leave at the next exit or service area if you can. If you can't, go left: into the left lane, an emergency area or the hard shoulder, and get out on the side away from the traffic. If you can't get out safely, stay in the car with your seat belt on and call 999.

That advice comes from Great Britain's Highway Code. Rule 277 is long, and worth reading in full rather than in two lines here.

Once the car's back, it's worth knowing which fuel it takes before the next fill. The handbook will say, and a petrol car made from 2019 on should have an E5/E10 label by the filler cap. Our free car check shows the fuel type on the car's record, along with the MOT history, tax status and a running-cost estimate, all from the number plate. Fuel is one of the biggest costs in what a used car costs to run.

Will breakdown cover or insurance pay?

Maybe. Two different policies are in play, and both depend on their wording.

Breakdown cover first. The RAC puts it plainly: some breakdown policies include assistance if you put the wrong fuel in, while others charge separately for draining or recovery. So the drain may be free, or it may be a bill on top of your membership. The only way to know is to read your policy document, and the time to do that is before you're stood at a pump.

Check where the cover applies, too. One Financial Ombudsman decision from January 2025 looked at an Amtrust breakdown policy that covered misfuelling "within the UK only", with recovery up to 10 miles. The driver had misfuelled abroad, so the claim wasn't covered, and the ombudsman didn't uphold the complaint.

That's one policy, not the market. But it shows the kind of limit that can sit in the small print.

Motor insurance is the second policy. Some insurers exclude misfuelling and some don't.

In a 2023 decision, Admiral relied on an exclusion for "any loss or damage caused by using the incorrect type of fuel". The driver had put AdBlue in, which the ombudsman held is an exhaust fluid and not a fuel, so that exclusion didn't apply and Admiral was told to pay.

In a 2017 decision, a Tradewise policy didn't exclude misfuelling at all. The claim still failed. The ombudsman preferred the independent engineer, who found mechanical failure rather than fuel damage, and the driver hadn't kept the receipt that would have shown what he'd bought.

So there are two questions for your insurer. Does the policy exclude the wrong fuel? And if it doesn't, can you show that's what caused the damage? Keep the receipt, tell the recovery crew what went in, and get it written on their job sheet.

Whether a claim touches your no-claims discount or next premium is a question for your insurer, and nothing I've read settles it either way. The same goes for hire and lease cars. Check the agreement before you call anyone.

If you've just bought the car, our guide to insuring a car you've just bought covers how to find your way round a policy booklet.

One last thing that isn't misfuelling. If you've put one tank of E10 petrol in an older car that isn't approved for it, that's a different event with different advice. Government guidance says it shouldn't need a drain. Our E10 compatibility guide covers it.

Common questions about the wrong fuel