With millions of people expected to be travelling on UK roads over the August Bank Holiday weekend, car leasing comparison website LeaseLoco, is warning motorists about the dangers of misfuelling.

What seems like an innocuous mistake could end up costing you thousands of pounds in repairs, particularly if you accidentally put petrol in your diesel car – which could lead to you having to replace the engine.

John Wilmot, CEO of LeaseLoco, said: “This Bank Holiday weekend is set to be a scorcher. But for some families it could end up being a damp squib, because of a lapse in concentration and an easily avoidable mishap.

“With over 400 motorists misfuelling every day, potentially more than 1,000 Brits will fill their cars up with the wrong fuel this weekend and face the prospect of a huge repair bill. Unfortunately, it’s also the most common mistake – putting petrol in a diesel car – which can result in the most damage.

“Petrol damages diesel engines more than vice versa. And it’s easier to accidentally put petrol into a diesel tank because petrol nozzles can fit into most diesel filter necks. My advice to motorists is concentrate and take your time while refuelling, and make sure you check the fuel grade indicator on the pump and the label on the pump trigger. Never rely on the colour of the hose or nozzle.”

What happens when you put petrol in a diesel car?

Petrol damages diesel engines because it increases friction between parts in the engine. Diesel, on the other hand, acts as a lubricant helping the fuel pump do its job.

The more petrol that’s pumped through the engine of a diesel car the more damage it will do. And the damage to fuel pumps, injectors, and the fuel tank could be severe. In the worst-case scenario, you may even need a new engine.

What happens when you put diesel in a petrol car?

It is far less common to put diesel in a petrol car because the diesel pump nozzle is bigger than most petrol filler necks.

Fortunately, diesel in a petrol engine is not fatal. The diesel will clog up the spark plugs and fuel system and will probably result in the car not starting. However, there shouldn’t be any lasting damage once the fuel has been drained out.

What should you do if you misfuel your car?

If you realise you’ve misfuelled your car, you should leave your engine switched off and not put the key in the ignition. The most serious damage from misfuelling occurs after starting the engine because this allows fuel to circulate around it.

The fuel tank needs to be drained to remove the contaminated fuel before you drive the car. But before you can do this you should put the car in neutral, let the staff at the petrol station know what’s happened, and push the vehicle to a safe place on the forecourt.

If you’ve already left the petrol station, find a safe place to pull over as soon as possible, put the car in neutral and turn off the ignition.

Most breakdown cover policies won’t cover the costs of draining and flushing your vehicle after misfuelling, but the big breakdown providers offer the service separately whether you’re a member or not. There are also plenty of specialist companies offering misfuelling services.

Check your insurance policy to see if misfuelling is covered. You may be able to claim back the cost of repairing your car under an accidental damage clause in a comprehensive policy.

To avoid misfuelling, check the fuel grade indicator on the pump and the label on the pump trigger. Never rely on the colour of the hose or nozzle.