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02 Aug 2021
Chris Torney Chris Torney

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


If you’ve put petrol in a diesel car – or vice-versa – the most important thing to do is NOT switch on the engine or even put the key in the ignition. We explain more below.

Petrol being pumped into a car

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

The risks attached to misfuelling have led the government to introduce new measures aimed at ensuring motorists don’t end up making this mistake.

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New petrol and diesel pump labels explained

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

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

Since then, petrol pumps have been fitted with a circular label with E5 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 E5 on petrol pumps refers to the 5% 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.

Just as useful for drivers concerned about putting the right type of fuel into their cars, petrol pumps are usually green while diesel pumps are black.

Over the course of 2021, the standard form of petrol provided by UK filling stations is changing from E5 to E10. This means it can contain up to 10% ethanol.

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.


What happens when you put petrol in a diesel engine?

There are thought to be around 150,000 instances of misfuelling in the UK every year. In the majority of cases, this involves motorists putting petrol into their diesel vehicles by accident.

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.

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, there’s no guarantee you’ll be able to recoup the costs under your car insurance.


What happens when you put diesel in a petrol engine?

It’s much less common for owners of petrol cars to put diesel into their fuel tanks by mistake.

Not because petrol drivers are savvier, but because the wider design of diesel pump nozzles means they don’t fit easily into the necks of petrol tanks.

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.

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.


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

The first thing to do as soon as you realise you have put the wrong fuel in your car 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.

Step 1: 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.

Step 2: It makes sense to move your car out of the way of other customers. If possible, put your vehicle 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.

Step 3: Call your car insurance provider and explain what’s happened.

Step 4: 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 will face a bill for doing so.


Breakdown and car insurance – are you covered?

Having car insurance and breakdown cover doesn’t mean that you can have your misfuelling mistake put right at no cost.

Usually, breakdown companies charge extra for emptying and flushing a tank, even to their own existing customers.

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

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 about your car insurance policy?

In some cases, motor policies could include cover for misfuelling, while others might allow you to claim for this kind of problem under accidental damage.

But this is certainly not the norm among insurance providers. 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.


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 will let you know soon enough.

As mentioned above, diesel in a petrol engine will usually not be able to ignite as it won’t have been compressed.

This means that your car’s engine won’t be able to run and your car won’t start.

Misfuelling by putting petrol in a diesel car is much more common – and the consequences can be much more dramatic.

Possible warning signs include jerky acceleration as the petrol damages the pumps in your car’s engine, as well as smoke coming out of your exhaust.

You might also experience your engine misfiring.


How to avoid putting the wrong fuel in your car

Aside from simply paying more attention when you are at a filling station, there is 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 permits the diesel nozzle to enter.

You can expect to pay between £20 and £40 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.


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