Diesel and petrol work differently, as do the engines designed for each of these fuels. So, the damage that happens to your car will depend on the engine and the fuel it's supposed to have.
When you put petrol in a diesel car
Putting petrol in a diesel car is the most common type of misfuelling as petrol nozzles easily fit into modern diesel cars. However, it's also the most serious type of misfuelling.
If it's not dealt with straight away, putting petrol in your diesel car will damage your fuel pump. Diesel helps the fuel pump do its job by acting as a lubricant, but when it's mixed with petrol it has the opposite effect and acts like a solvent. The loss of lubrication will cause the metal parts to come into contact and rub together. This means it will do more damage the more it's pumped through your fuel system. This type of misfuelling can even result in needing a new engine.
HDi or common rail diesel engines can suffer particularly bad damage as a result of misfuelling, which could mean you need to look at new fuel pumps, injectors, pipes, filters and fuel tank.
When you put diesel in a petrol car
As the petrol filler necks on most cars are much smaller than the diesel nozzle, this type of misfuelling is not as common. Whilst putting diesel in a petrol car doesn't have the potential to cause as much damage as putting petrol in a diesel car, it can still cause serious damage. This type of misfuelling is particularly harmful to direct injection (GDI) petrol engines.