It is an easy mistake, and a surprisingly common one. You are distracted, you are driving an unfamiliar or hire car, or you simply reach for the pump you always use, and only afterwards realise you have filled a diesel car with petrol, or a petrol car with diesel. According to the RAC, around 150,000 drivers in the UK put the wrong fuel in their car each year.
The most important thing is also the simplest: do not start the engine. What happens next depends almost entirely on whether the wrong fuel has been allowed to circulate through the fuel system. If the engine has not been started, the problem is usually confined to the tank and can be put right by draining it. If the engine has run, the wrong fuel has already travelled further, and the repair can be far more involved.
This guide covers what to do at the filling station, what to do if you have already driven off, why petrol in a diesel is the more damaging mistake, and what a drain and flush involves. It does not tell you how to drain a tank yourself. That is a job for a specialist or a garage, not a roadside DIY fix.
If you are in danger, or broken down in a live lane, call 999 first. Do not work on the car in moving traffic.
1. If you realise at the pump: do not start the engine
If you are still at the filling station and you realise you have used the wrong pump, stop. Do not switch the ignition on, do not put the key in the ignition, and do not press the start button. Switching the ignition on can prime the fuel pump and start moving fuel around the system, even before the engine turns over.
Instead:
- Leave the car where it is for the moment, with the handbrake on.
- Tell a member of staff at the filling station what has happened. They will have a procedure for this, and they need to know before the car is moved.
- Put the car in neutral and, if it is safe to do so and staff direct you to a safe place, push it away from the pumps. Do not try to push it into moving traffic, and do not ask other drivers to tow it.
- Call for help: your breakdown provider, your insurer, or a wrong-fuel specialist, and tell them exactly what happened: which fuel went in, how much, and that the engine has not been started.
Do not try to fix it by topping the tank up with the correct fuel. Petrol and diesel mix in the tank; adding the right fuel on top does not undo the contamination, and it can make the drain harder. Do not try to siphon fuel out of the tank yourself either. Fuel is flammable, and a DIY drain risks spills, fumes and a tank that is still contaminated.
2. Petrol in a diesel car is the more serious mistake
Most misfuelling is petrol put into a diesel car. Petrol pump nozzles are smaller than diesel nozzles, so a petrol nozzle fits easily into a diesel car’s filler neck. The reverse is much less common, because most diesel nozzles will not fit into a petrol car’s filler neck.
It is also the more damaging direction. Diesel fuel is oily and helps lubricate the finely machined moving parts inside a diesel fuel pump. Petrol acts as a solvent: it strips that lubrication away, so metal parts rub against each other, wear, and shed tiny metal particles that are then pumped around the rest of the fuel system. Petrol also burns differently from diesel, which can overheat pistons and the cylinder head. That is why running a diesel car on petrol can damage the pump, injectors, filters and fuel lines, and in the worst cases the engine itself.
A small amount still matters. Even a few litres of petrol mixed into a tank of diesel changes the lubrication enough to matter, so “only a little” is not a reason to drive on and hope. The tank needs draining whether you added five litres or fifty.
3. Diesel in a petrol car: less damaging, still not driveable
Putting diesel into a petrol car is less likely, and usually less destructive, but the car still cannot be driven. Diesel does not ignite from a spark plug the way petrol does, so a petrol engine running on diesel will misfire, produce heavy smoke, lose power and cut out, and it may not restart at all. Diesel can foul the spark plugs and clog the injectors and filter.
The advice is the same: do not start the engine if you have not already done so, tell the filling station staff, move the car only by pushing it to a safe place, and call for a drain and flush. The system still needs to be emptied and cleaned before the car is driven again, even though the damage is more often cleanable than structural.
4. If you have already started the engine or driven off
Do not panic, but act quickly. The less time the engine runs on the wrong fuel, the less of the fuel system it reaches.
- Turn the engine off immediately if it is safe to do so. If you are moving, pull over to a safe place first, switch the engine off, and do not restart it.
- Put the car in neutral, apply the handbrake, and switch on your hazard lights.
- Call for help and say that the engine has already run, so whoever attends knows the fuel has circulated. Tell them what symptoms you noticed, if any, and how far you drove.
- Call your insurer as well as your breakdown provider. Misfuelling is not covered as standard by every policy, so check what your policy says and follow your insurer’s instructions. Do not assume the drain or any repair will be paid for.
The RAC lists the symptoms to watch for. With petrol in a diesel car, you may notice a loud knocking sound when accelerating, excessive smoke from the exhaust, slower acceleration than usual, the engine warning light coming on, the engine stopping altogether, or the car struggling to restart. With diesel in a petrol car, you may notice misfiring, excessive smoke, the engine cutting out, or the engine failing to restart. Any of those after filling up means: stop, switch off, and do not restart.
Even if the car seems to be running normally, do not keep driving to “see if it clears”. Damage from petrol in a diesel system builds as the contaminated fuel circulates, and a car that runs for a short while can still need a full fuel-system repair afterwards.
5. What happens next: drain, flush, and recovery if needed
A proper wrong-fuel job is not just emptying the tank. A specialist or garage will drain the tank and the fuel system of contaminated fuel, flush the lines and filter with clean fuel, and add enough of the correct fuel to get the car moving again. If the engine has already run and the fuel system is damaged, the car may need to be taken to a garage instead, and parts such as the pump, injectors or filter may need attention or replacement.
What you will need to tell the person who attends:
- where you are, including the filling station name or road name and postcode if you have moved
- your vehicle make, model and registration
- which fuel you put in, roughly how much, and whether the engine has been started or the car driven
- whether the car is still on a filling-station forecourt, and whether staff have directed you to a particular spot
Keep the receipt from the filling station if you have it, and note the time you filled up. Your insurer may ask for both.
Do not let anyone talk you into driving the car to a garage “because it is only a short distance”. If the engine has not been started, driving it there starts the very circulation you have been trying to avoid. If it has already run, driving it further only spreads the contamination. The car should be drained where it is, or recovered to where the work can be done.
6. How to avoid it next time
Most misfuelling happens when attention slips. A few habits help:
- Check the pump label and the filler cap before you lift the nozzle, especially in an unfamiliar car, a hire car, or a car you do not normally drive.
- Do not fill up while distracted by a phone call or a conversation. If you are interrupted mid-fill, check again before you resume.
- If you switch between petrol and diesel cars, or you have just changed cars, say the fuel type out loud before you start. It sounds obvious, and it is the check that catches the mistake.
- If a car has a misfuelling prevention device on the filler neck, do not force a nozzle that does not fit. A nozzle that will not go in easily may be telling you it is the wrong fuel.
Need recovery now?
UK Breakdown Recovery is a London-based, 24/7 vehicle recovery service for cars, vans, motorbikes and more. If you have put the wrong fuel in your car and you are in a safe place, call or WhatsApp +44 7588 282876 for wrong-fuel drain / flush or recovery, and tell us where you are, what fuel went in, and whether the engine has been started. Do not start the engine while you wait.
If you are in a live lane, or anyone is hurt, call 999 first.
FAQ
Can I drive a short distance with the wrong fuel in the tank?
No. If the engine has not been started, starting it circulates the wrong fuel and turns a tank drain into a much bigger repair. If it has already run, stop as soon as it is safe to do so and do not restart it. The car should be drained where it is, or recovered.
I only put a little petrol in my diesel car. Does it still need draining?
Yes. Petrol mixes with diesel in the tank and changes the lubrication the fuel pump relies on, even in small amounts. Do not try to dilute it by topping up with diesel. The tank needs draining and the system flushing.
Is diesel in a petrol car as bad as petrol in a diesel car?
No. It is usually less damaging, because diesel fouls plugs and injectors rather than stripping lubrication from a high-pressure pump. But the car still must not be driven, and the tank and system still need draining and flushing before it is used again.
Will my insurance cover misfuelling?
Not always. Some policies cover it as part of accidental damage and many do not cover it as standard. Check your policy documents and call your insurer as soon as you can, and follow their instructions about who should attend.
What if I have already driven off and the car has stopped?
Switch the engine off if it is still running, do not try to restart it, move to a safe place if you can do so without driving, and call for help. Tell whoever attends how far you drove and what symptoms you noticed, which helps them judge how far the wrong fuel has circulated.
