Put the Wrong Fuel In? Here’s What To Do Next

The single most important rule: do not start the engine

If you realise the mistake at the pump, before turning the key, the fuel is sitting in the tank and filler neck only, not in the fuel lines, pump or injectors. This is by far the easiest and cheapest situation to put right: a technician can drain the tank and refill it with the correct fuel, usually without any further work needed.

The moment the engine is started and fuel is drawn into the system, the risk and potential cost of the mistake increases significantly, because the wrong fuel is now reaching components that rely on the correct fuel's properties to work and to protect themselves.

Why petrol in a diesel is the more serious mistake

Diesel fuel injection systems, particularly modern common-rail systems, rely on the diesel itself to lubricate the high-pressure fuel pump and injectors as it passes through them. Petrol does not have this lubricating quality, and it also has different combustion characteristics, so running a diesel engine on petrol, even for a short distance, can cause serious wear or damage to the fuel pump and injectors, which are expensive components to replace.

Diesel in a petrol engine is generally considered less immediately damaging in small quantities, since petrol engines are less dependent on the fuel for lubrication, but it should still be treated the same way: do not run the engine, and get it professionally drained rather than driving on it.

Don't try to dilute it yourself

A common instinct is to simply fill the tank with the correct fuel to dilute the mistake down to a 'safe enough' ratio. In practice this rarely gets the mixture to a genuinely safe level, particularly for diesel-in-petrol-tank scenarios, and it leaves you with a much larger volume of contaminated fuel that still needs to be professionally removed, at a higher cost than if you had simply called for a drain straight away.

What a professional drain and flush involves

A technician attends with specialist equipment to safely drain the tank of the wrong fuel, and where the engine has already run, to flush the fuel lines through as well, before refilling with the correct fuel. This is a roadside or driveway job, not something that requires the vehicle to be taken off to a workshop first, and it is considerably cheaper than replacing a damaged fuel pump or set of injectors.

If the vehicle will not restart cleanly after a drain and flush, or a fault develops afterwards, it may need recovery to a garage for further investigation, but in the majority of cases where the mistake is caught before the engine is run for any distance, a straightforward drain resolves it.

If you've already been driving

If you only notice something is wrong after driving for a while, for example a loss of power, unusual noises, or the engine running roughly, stop as soon as it is safe to do so and turn the engine off. Do not attempt to restart it repeatedly to see if it clears, since this simply draws more of the wrong fuel through the system. Arrange for the vehicle to be recovered to a garage rather than continuing to drive it.

How to avoid misfuelling in the first place

Misfuelling most often happens when a driver is rushed, distracted, or using an unfamiliar vehicle, such as a hire car, a courtesy car, or a partner's vehicle whose fuel type is different from their own. Taking a moment to check the fuel type, shown on the fuel filler cap or just inside it on almost all modern vehicles, before lifting the nozzle takes only a few seconds and removes the risk entirely.

Some drivers develop the habit of always double-checking the nozzle colour and label at the pump, since diesel and unleaded nozzles can be positioned close together and, especially in poor light, are easy to reach for on autopilot.

What happens to the fuel that's drained out

Once safely drained from your vehicle, the contaminated fuel is disposed of or recycled appropriately by the technician, in line with standard waste fuel handling practices; it is not simply reused or put back into general circulation.

This is one of the reasons a professional drain and flush costs money rather than being something to attempt yourself: it requires the correct equipment to safely remove the fuel from the tank and lines, and to dispose of it properly, rather than simply syphoning it out.

What to tell the recovery operator when you call

When you call for a misfuel drain, it helps to have a few details ready: which fuel your vehicle should take, which fuel was actually put in, roughly how much was added before you realised, and whether the engine has been started or run at all since. This lets the operator bring the right equipment and gives them a clearer picture of how straightforward the job is likely to be.

If you are at a fuel station forecourt, let a member of staff know what has happened; many stations are used to dealing with this and may be able to help you move the vehicle to a safe spot to wait, away from the pumps, until the recovery technician arrives.

Misfuelling in a hire car, courtesy car or company vehicle

Misfuelling is particularly common in vehicles you do not normally drive, since the fuel type is not something you check automatically the way you would with your own car. If you are driving a hire car, a courtesy car, or a company vehicle and are not certain of its fuel type, take a moment to check the fuel filler cap or the vehicle handbook before refuelling, especially if it is a make or model you are not familiar with.

If a misfuelling does happen in a hire or courtesy vehicle, contact the hire company or garage that provided it as well as arranging a drain, since there may be a specific process they want followed and their own insurance or damage waiver may be relevant to how the cost is handled.

A quick reference for the moment it happens

Stop filling as soon as you realise. Do not start the engine, or if it's already running, turn it off as soon as it's safe to. Do not top up with the correct fuel to dilute it. Call for a drain and flush, and tell them the fuel type, how much went in, and whether the engine has run. That's usually enough to get you sorted with no lasting damage.

Why this is more common than you'd think

Fuel filler caps, nozzle colours and pump layouts vary between stations, and a moment's distraction, especially in an unfamiliar car, is all it takes. It happens to careful, experienced drivers regularly, and in the vast majority of cases, caught early, it is a fully recoverable mistake rather than a lasting problem.