Wrong fuel in the car? Do not start the engine
Misfuelling is one of the most common and most survivable mistakes a driver can make — but only if you catch it before the key turns. What it costs depends almost entirely on the next sixty seconds.
The one thing that matters
Do not start the engine. Not to move the car, not to see what happens, not for a second.
While the wrong fuel sits in the tank, it is a draining job. The moment you turn the key, the fuel pump draws it through the whole system — pump, filter, lines, injectors — and a straightforward drain becomes a repair. Even turning the ignition to the position that primes the pump can be enough on some cars.
- Do not turn the key. If you already have, switch it off immediately — every second of running spreads contamination further.
- Tell the forecourt staff. They deal with this regularly and can help you move the car safely.
- Put it in neutral and push it clear of the pumps rather than driving it.
- Do not smoke, and keep the area clear.
- Call for a fuel drain. The sooner it is done, the less risk of damage.
- Keep the receipt. It proves what went in and how much, which the person draining it needs to know.
Petrol in a diesel is the serious one
The two mistakes are not equivalent, and it is worth knowing which one you have made.
Petrol in a diesel car
This is the damaging direction, and it is also the more common one because a petrol nozzle fits easily into a diesel filler neck.
Diesel lubricates the fuel pump and injectors as it passes through. Petrol does the opposite — it acts as a solvent and strips that lubrication away. Metal parts machined to very fine tolerances then run dry against each other, and wear accelerates quickly. Run the engine on it and you can be looking at a new pump, new injectors, or in bad cases far worse.
Diesel in a petrol car
Less serious, and harder to do by accident because a diesel nozzle is usually too large for a petrol filler. Diesel is heavier and oilier and does not ignite readily in a petrol engine, so the usual outcome is a car that runs very badly, misfires, smokes and stops. At worst the fuel system and tank need draining and cleaning.
In both cases the same rule applies. Whether it is expensive or merely inconvenient is decided by whether the engine was started.
How a fuel drain works
- The tank is emptied of the contaminated mixture, usually at the roadside or on the forecourt.
- The system is flushed so nothing is left in the lines.
- Fresh fuel goes in — enough to run properly and get you to a filling station.
- The car is started and checked before you drive away.
- If the engine was run, a filter change or further work may be needed, and that is judged case by case.
A drain on a car that was never started is a routine job. The variable that changes the bill is not the make of car or the amount of fuel — it is whether it ran.
Contaminated fuel is waste and has to be disposed of properly. It is not something to drain onto a forecourt or into a drain, which is another reason this is not a DIY job.
Why it happens to careful people
Misfuelling is not carelessness so much as autopilot. It happens most often when something breaks the routine.
- A different car. A hire car, a courtesy car or a partner’s car after years of driving the opposite fuel.
- A change of vehicle. The first few months after switching from diesel to petrol, or the reverse.
- Distraction. On the phone, tired, children in the back, running late.
- An unfamiliar forecourt layout, or pumps where the colour coding is not what you expect.
The simplest defences are a label inside the filler flap and a habit of reading the nozzle before it goes in rather than after. If you have just changed cars, it is worth saying the fuel type out loud for the first few fills — it feels silly and it works.
Leave the key alone and call us
If the engine has not been started, this is usually a straightforward job and your car will be fine. We come to you across North Wales, on the forecourt or wherever the car is. Tell us the fuel type, roughly how much went in, and — honestly — whether the engine was started, because it changes what we bring.
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Questions, answered
What should I do if I put the wrong fuel in my car?
Do not start the engine. Tell the forecourt staff, put the car in neutral and push it somewhere safe, then call for a fuel drain. While the wrong fuel sits in the tank it is a draining job; starting the engine draws it through the pump, filter, lines and injectors and turns it into a repair.
Is petrol in a diesel car serious?
It is the more damaging of the two. Diesel lubricates the fuel pump and injectors, and petrol acts as a solvent that strips that lubrication away, so finely machined metal parts run dry and wear quickly. If the engine is run on it, you can be facing a new pump or injectors.
What happens if I put diesel in a petrol car?
It is less serious. Diesel is heavier and does not ignite easily in a petrol engine, so the car usually runs very badly, misfires, smokes and stops. At worst the fuel system and tank need draining and cleaning. It is also harder to do by accident, because a diesel nozzle is normally too large for a petrol filler.
I already started the engine. What now?
Switch it off immediately and do not restart it. Every second of running spreads contamination further through the system. The car will still need draining, and depending on how long it ran, a filter change or further work may be needed.
How much does it cost to drain the wrong fuel?
It depends mainly on one thing: whether the engine was started. A drain on a car that was never started is a routine job. Once the engine has run, the cost depends on how far the contamination spread and what needs replacing, so it is judged case by case.
Can I drain the fuel myself?
It is not advisable. Contaminated fuel is hazardous waste that must be disposed of properly rather than tipped onto a forecourt or into a drain, and the system needs flushing rather than just emptying. Getting it wrong risks leaving contamination in the lines.
Will my insurance cover misfuelling?
Some policies cover it and some do not, and it may be an optional extra. Check your policy documents. Keep the fuel receipt either way, as it proves what went in and how much, which both the insurer and whoever drains the tank will want to know.