Putting the wrong kind of fuel in your vehicle is a more common mistake than most people would believe: as many as 150,000 people put the wrong fuel in their car every year. This is an especially common problem for those who frequently hire cars or own more than one vehicle, but it can cause serious damage if handled incorrectly.

The most important thing if this ever happens to you is to never turn on the ignition or start the engine. Turning on the ignition of a modern petrol or diesel car will often prime the fuel pump, meaning that fuel is already beginning to circulate throughout the fuelling system. With the complexities of modern fuel injection systems, it’s a difficult problem to fix.

A diesel fuel pump is lubricated by the fuel itself and many seals will be damaged by petrol. This will also have a retroactive effect on the lubrication of the pump. If you have a modern common rail or HDi diesel engine and the fuel reaches the rail system, it can often be considerably cheaper and easier to fit a new fuelling system entirely. To avoid this expensive result, don’t turn the ignition on - even if it means you can’t listen to the radio while you wait to get the problem fixed!

It is worth noting that putting petrol in a diesel tank can invalidate your manufacturer’s warranty - especially while the engine is in the running-in phase. However, as a general rule, it is worth considering that a little amount of petrol in diesel engines won’t damage the engine. If the level of petrol in the fuel tank reaches more than 10%, it will be necessary to drain the lot and refill it with diesel. Catch the mistake early, with less than 10% petrol to diesel, and it is possible to fill the tank all the way up with diesel and head on your way. However, before you go anywhere it is important to ask your local dealership for advice.

Fortunately, the same rule applies to petrol engines as well. If you happen to fill your petrol tank with diesel and there is more than 10% diesel in the petrol tank, the car should not be run and the tank will need to be drained. With less than 10% of the wrong fuel type, simply fill the car with as much petrol as you can and you should negate any potential damage. It is worth noting that the increased diameter and significantly different colour of a typical UK diesel pump make it much harder to make the mistake of filling a petrol-run car with diesel. If you catch yourself struggling with the pump and the nozzle doesn’t seem to want to fit, make sure you’ve got the correct type of fuel pump in your hand.

Regardless of whether or not you’ve put diesel into a petrol engine, or petrol into a diesel engine, always speak to the manufacturer before following the above guidelines. Some modern models or much older cars will not be quite as tolerant of your mistake as other vehicles. The best port of call is your local dealer, who will usually be happy to give you advice relating to your specific car model.