What to do if you misfuel
According to the AA, around 150,000 UK drivers put the wrong fuel in their car every year: that's one every three and a half minutes. If it happens to you, the most important thing is to stay calm and act quickly. The damage is directly proportional to how far you drive with the wrong fuel in the tank.
The most common mistake: petrol into a diesel
Putting petrol into a diesel car is by far the most common misfuelling error, and it's easy to understand why. The petrol pump nozzle is narrower than a diesel nozzle and fits easily into a diesel filler neck. Diesel acts as a lubricant for the fuel pump and injectors. Petrol strips this lubrication, meaning even a short drive can cause metal-to-metal contact and damage the fuel pump, a component that can cost £1,000–2,500 to replace.
The less common: diesel into a petrol
Putting diesel into a petrol car is rarer because the wider diesel nozzle is designed not to fit into a petrol car's narrower filler neck. It can happen with older cars or if someone forces the nozzle. If it does occur, diesel in a petrol engine typically causes the car to run very poorly and produce black smoke, but is generally less immediately damaging than the reverse. The car will usually cut out before serious engine damage occurs.
Exactly what to do
This is the single most important step. If you realise your mistake at the pump (before starting the car), you've avoided the most costly outcome. Even if you started the engine briefly and stopped quickly, do not restart it.
Move your car away from the pump without starting the engine. Use neutral and push it, or ask for assistance. Alert the forecourt staff.
AA, RAC and Green Flag all offer misfuelling assistance. Specialist fuel drain companies are also widely available and may respond faster. A drain typically costs £150–400 and takes 30–60 minutes.
The technician will drain the contaminated fuel, flush the system, and refill with the correct fuel. In most cases where the engine hasn't been started, no permanent damage will have occurred.
Stop as soon as it's safe to do so. The longer you drive, the more the contaminated fuel circulates through injection components. Call your breakdown provider immediately. Be honest about what happened. They need to know to advise correctly. Do not add more of the correct fuel hoping to dilute the problem. It won't help and may make recovery harder.
Prevention: how to avoid it
- Look at the nozzle colour: petrol nozzles are typically black or green; diesel is typically black or yellow. Always check before inserting.
- Label your fuel cap: a simple sticker inside the fuel flap saying "DIESEL" or "PETROL" takes seconds to add and has prevented thousands of mistakes.
- Misfuelling prevention devices: products like Fuel Angel insert into the filler neck and only accept the correct nozzle size. Useful for fleet vehicles or particularly absent-minded drivers.
- Most risk when driving a hire car or unfamiliar vehicle. Always double-check the fuel type of any vehicle that isn't yours before filling up.