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What to do if you put the wrong fuel in your car

If you’ve put petrol in a diesel car – or diesel in a petrol car – the most important thing to do is NOT switch on the engine or even put the key in the ignition.

Putting the wrong fuel into your car can be one of the most damaging things you can do to your vehicle.

The risks attached to misfuelling are so great the government has introduced measures aimed at ensuring motorists don’t end up making this mistake. Here’s what you need to do.

A driver fills up their car with fuel at a petrol station 
 

Why is putting petrol in a diesel car so damaging and vice versa?

Your car has been engineered to use a specific type of fuel. And the properties of petrol and diesel are different enough that they have the potential to damage to your car's engine, though petrol can do much more damage in a diesel engine than the other way around.

 

    What happens when you put petrol in a diesel engine?

    If you turn on the engine of a diesel car when it has petrol in it, the petrol can act as a solvent. This means it could cause damage to parts of the fuel system such as the fuel pump.

    This is because diesel, as well as being used as a fuel, is also a lubricant. It reduced friction and keeps the engine running smoothly. Petrol is the opposite - it increases friction. This causes additional wear on your diesel engine and could eventually cause parts to fail.

    In the most serious cases, you might have to have your car’s fuel system – including the fuel tank and filters – totally replaced, at considerable expense.

    And although some insurance providers include this sort of repair, there’s no guarantee you’ll be able to recoup the costs under your car insurance.

    There are thought to be around 150,000 instances of misfuelling in the UK* every year where motorists put the wrong fuel in their car. In the majority of cases, this involves motorists putting petrol in diesel cars by accident.

     

    What happens when you put diesel in a petrol engine?

    If you do manage to get some diesel into your petrol engine, the consequences are likely to be less severe than putting petrol into a diesel car.

    Diesel fuel needs to be compressed before it ignites and drives the engine, so in most cases your car simply won’t be able to start.

    It’s much less common for owners of petrol cars to put diesel into their fuel tanks by mistake anyway. This is because the wider design of diesel pump nozzles means they don’t fit easily into the necks of petrol tanks.

     

    Put the wrong fuel in your car? What to do

    If you put the wrong fuel in your car, follow these steps:

    • Do nothing.  If you realise you’ve put the wrong fuel in your car, the best thing you can do is nothing. Don’t try to start the engine. In fact, don’t even put the key in the ignition. Misfuelling is a much more straightforward problem to fix if the petrol or diesel hasn’t yet left the tank and entered your car’s fuel system.
    • Own up. Hopefully you’ll have realised your error while you’re at the petrol station and before you’ve driven off. So speak to staff there and tell them what you’ve done.
    • Move your car without turning it on. It makes sense to move your car out of the way of other customers. If possible, put your car into neutral or take it out of gear. Then push it – with help from your passengers, other customers or petrol station staff – to a safer location on or near the forecourt.
    • Call your car insurance provider and explain what’s happened.
    • Contact a breakdown firm and tell them what has happened. They should be able to drain your fuel tank and get you back on the road, especially if you haven’t tried to turn your engine on. Don’t worry if you don’t have breakdown cover -  you can still ask the breakdown company to help, but you might face a bill for doing so.
     

    I’ve put petrol in a diesel car - what do I do?

    If you put petrol in a diesel car the most important thing to do is nothing. Don’t attempt to drive your car or even start the engine.

    As soon as you start the engine, the petrol starts flowing around it and could wreak havoc on the fuel system.

    Instead, follow our top tips:

    • Seek help from the petrol station staff
    • Push your car to a safe location
    • Calling your insurance company 
    • Call a breakdown service who can help get you back on the road
     

    What do I do if I put petrol in a diesel car and drive it?

    The chances are if you put petrol in a diesel engine and then drive it, you’re likely to break down. If you did manage to get home your car probably won’t start the next morning.

    If you break down or your car won’t start, call your insurance company. Then call a breakdown service or fuel draining company. If you repeatedly try to start your car you might find you end up with a flat battery too.

     

    Is misfuelling covered by car insurance?

    In some cases, car insurance policies include cover for misfuelling, while others might allow you to claim for putting the wrong fuel in your car under accidental damage.

    But this is certainly not the norm. This is why it’s a good idea to contact your insurer as soon as you realise you’ve put the wrong fuel in your vehicle.

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    Can I call my breakdown provider after misfuelling?

    Yes. Whether you have breakdown cover or not, you can call a breakdown service for help if you put the wrong fuel in your car. 

    Usually, breakdown companies charge extra for emptying and flushing a tank, even to customers with breakdown cover.

    In some cases, you might be able to select a drainage service as an optional extra when you get your breakdown cover policy.

    As an alternative, you might be able to get your breakdown company to tow your vehicle to a garage in order to deal with the problem.

    You can compare garages that offer misfuelling services to ensure you get the best deal. If the damage is severe enough, you might have no option but to leave your car with a mechanic.

     

    What are the symptoms of putting the wrong fuel in your car?

    If you don’t realise you’ve put the wrong fuel in your car straight away, your vehicle should let you know soon enough. 

    Put diesel in a petrol engine and your car simply won’t start - this is because the engine won’t ignite as the fuel can’t be compressed.

    If you put petrol in a diesel car, warning signs can include:

    • Jerky acceleration as the petrol damages the pumps in your car’s engine
    • Smoke coming out of your exhaust
    • The engine misfiring.
     

    How do I avoid putting the wrong fuel in my car?

    Aside from paying more attention when you’re at a filling station, there’s one way of ensuring that you don’t put petrol in your diesel car.

    This involves buying a device that prevents petrol fuel nozzles from fitting into diesel tank necks, but which still lets the diesel nozzle enter.

    You can expect to pay between £5 and £50 for such a device, but make sure it’s compatible with your make and model of car before you buy.

    Given the potential costs and hassle involved with misfuelling, this could be worth considering.

     

    How do the new petrol and diesel pumps prevent misfuelling?

    In 2019, the Department for Transport announced that all fuel pumps in the UK were to get new labels.

    These should make it crystal clear which type of fuel – petrol or diesel – they deliver.

    To avoid putting the wrong fuel in your car, it’s also worth remembering that petrol pumps are usually green while diesel pumps are black.

    Since 2019, then, petrol pumps have also been fitted with a circular label with E10 written inside it.

    Diesel pumps have a square label with rounded corners and B7 inside.

    These codes are used to show the extent to which each fuel comes from more sustainable sources.

    The E10 on petrol pumps refers to the 10% of renewable ethanol that’s blended into petrol in the UK.

    Meanwhile, the B7 on diesel pumps means that up to 7% of biodiesel is used in normal diesel.

    Biodiesel comes from sources such as reused cooking oil or animal fats, whereas normal diesel – like petrol – is made from crude oil extracted from the earth.

    The government says that the vast majority of cars currently on the road in Britain are compatible with E10 fuel. But those that aren’t will have to use the more expensive ‘super’ grade petrol when filling up.

    The government’s website allows you to check whether your car is compatible with E10 petrol.

    Since 2020, new cars have had similar labels put on their fuel caps to indicate to owners what type of fuel they can use. And what percentage of it can be made up of either ethanol or biodiesel.

    But from 2030, new petrol and diesel cars will no longer be allowed to go on sale in the UK. This is part of the government’s attempts to meet its emissions targets.

     

    * Figures correct as of 1 August 2022