What Actually Happens If You Put the Wrong Fuel in Your Car

• If you have put the wrong fuel in your car, do not start the engine or even switch on the ignition: a tank drain on the forecourt typically costs £150 to £300 (about $200 to $400), while running the engine on the wrong fuel can lead to repair bills of up to £5,000 (about $6,700).
• Petrol in a diesel car is the more serious mistake, as petrol strips away the lubrication the diesel fuel pump and injectors rely on.
• Most standard car insurance policies exclude misfuelling, so check your breakdown cover and policy wording before you pay a garage.

Put the Wrong Fuel in Your Car? Leave the Key Out, Tell the Forecourt and Call for a Drain

The short answer: if you have just filled up with the wrong fuel and have not started the car, the fix is usually a fuel drain and flush on the spot, and your engine will come through unharmed. Leave the key out of the ignition, tell the staff at the petrol station, push the car to a safe spot if the forecourt asks you to, and call your breakdown provider or a specialist wrong fuel service. The RAC says the most serious damage from misfuelling happens after the ignition is switched on, so the moment you stop and do nothing is the moment you save money.

You are far from alone. The RAC puts the frequency of misfuelling in the UK at around once every three minutes, and Allianz estimates that around 150,000 people misfuel their vehicles each year. It happens most to drivers who switch between a petrol and a diesel car, drive a hire car or courtesy car, or simply fill up on autopilot at the end of a long day.

What happens next, and what it costs, comes down to two questions: which fuel went into which tank, and whether the engine has run on it.

What Happens If You Put Petrol in a Diesel Car

This is the more common and the more expensive mistake. The standard petrol nozzle is narrower than a diesel nozzle, so it slides into a diesel filler neck with no resistance at all. Allianz notes this is the reason petrol in a diesel car happens more often than the reverse.

Diesel fuel does more than burn. It also lubricates the high pressure fuel pump and the injectors, which run at extremely tight tolerances. Petrol acts like a solvent. Once it mixes with diesel it thins and dissolves that lubricating film, and if you switch on the ignition the fuel pump starts circulating the mixture through the lines, filter, pump and injectors. The RAC warns this increases friction between components and can damage the fuel lines and pump.

If you have already driven off, the signs tend to show up within a few miles. The RAC and Allianz list the typical symptoms as:

• a loud knocking or rattling from the engine
• heavy exhaust smoke
• the engine misfiring, losing power or stalling
• the car refusing to restart after it cuts out

If any of these appear, pull over somewhere safe as soon as you can, switch the engine off and do not try to restart it. Every extra mile pushes contaminated fuel further through the system.

How much petrol is too much? The RAC says that once petrol makes up more than about three percent of the fuel in a diesel tank, you can expect reduced performance, stalling and possible engine damage. In practice, a few seconds on the wrong pump in a near empty tank can pass that point, so the safe assumption is to drain it regardless of how little went in.

What Happens If You Put Diesel in a Petrol Car

Diesel in a petrol car is less common, mainly as the wider diesel nozzle does not fit most petrol filler necks easily. It is also less damaging, though it still needs sorting before you drive anywhere.

Petrol engines use spark plugs to ignite a fine mist of fuel. Diesel is thicker and oilier and does not ignite the same way. The RAC explains that once you start the engine, diesel coats the spark plugs and the fuel system, which often leads to misfiring. The engine can smoke, cut out or fail to start at all. Allianz advises that diesel will clog the fuel system and that the car should not be started, but drained instead.

If you have run the engine for a short time, the damage is often limited to a drain, a flush, and sometimes new spark plugs and a fuel filter. It is still a job for a professional, not something to try to drive through by topping up with the right fuel.

What Misfuelling Costs to Fix, and Whether Insurance Pays

The price depends almost entirely on whether the engine has run on the wrong fuel.

Engine not started. Allianz puts the typical cost of draining a fuel tank at £150 to £300 (about $200 to $400). RAC members still pay an extra fee for a roadside drain, as it is not part of standard breakdown membership. The RAC says its specialist fuel patrols drain the tank and fuel system on the spot and leave you with enough clean fuel to get moving again, and it has previously reported that drivers are back on the road on average 40 minutes after the patrol arrives. How long the drain itself takes varies with the car: the RAC notes that modern sealed tanks can stretch the job from a few minutes to a few hours.

Engine started or driven. Costs climb quickly. Allianz warns that engine damage from contaminated fuel can cost thousands to fix, with some cases reaching up to £5,000 (about $6,700). The top end of that range usually means a new high pressure fuel pump, injectors and fuel lines on a modern diesel, and Allianz notes that petrol in a diesel can leave you needing to replace the whole fuel system. Even at the lower end, expect the drain itself plus a new fuel filter, and on a petrol car possibly new spark plugs.

Insurance. This is where many drivers get caught out. Most standard car insurance policies treat misfuelling as driver error and exclude it, or exclude mechanical damage that results from it. Some insurers do include it. Allianz, for example, says its Online Silver and Gold cover levels pay to drain and flush the tank and repair engine damage caused by misfuelling, subject to an accidental damage excess. AXA offers wrong fuel cover as an optional add-on, capped at £250 (about $335) per claim, with anything above that down to you. Some breakdown policies also include misfuel assistance or sell it as an upgrade.

Before you agree to any work, ring your insurer and breakdown provider with your policy number to hand. A claim for a £200 drain can cost you more in lost no claims discount and excess than paying it yourself, so ask what a claim would do to your renewal before you make one.

Company car, lease or hire car drivers should call the fleet or hire company first, as the agreement usually sets out who pays for a misfuel and which recovery service to use.

E10, Hire Cars and How to Stop It Happening Again

E10 petrol, the standard grade at UK pumps, contains up to 10 percent renewable ethanol. Allianz says about 95 percent of petrol cars can use it, and every car built from 2011 onwards is compatible. If your older petrol car is not compatible and you fill it with E10 by mistake, there is no need for a drain: Allianz advises using up about a third of the tank and then topping up with E5 super unleaded. You can check whether your car is compatible using the GOV.UK E10 vehicle checker.

E10 in a diesel car is a different matter. It is still petrol, so it causes exactly the same damage as any other petrol misfuel and needs the same drain.

A few simple habits cut the risk sharply:

• Read the label on the nozzle before you lift it, not after. Green handles usually mean unleaded and black usually means diesel in the UK, but handle colours are not guaranteed, above all at supermarket forecourts and abroad.
• Check the sticker inside your fuel flap. If there is not one, add one.
• Take extra care in a hire car, a courtesy car or a second family car that runs on a different fuel from the one you normally drive.
• Fit a misfuelling prevention cap. These replace the standard filler cap and physically block a petrol nozzle from entering a diesel filler neck. Allianz recommends them for anyone who switches between cars regularly.
• Avoid filling up while on the phone or distracted by passengers.

If you have noticed your fuel economy dropping for no obvious reason after a fill up, it is also worth reading our guide on why your fuel economy is getting worse, as contaminated or wrong grade fuel is one possible cause.

When You Can Drive and When You Must Not

To keep it simple, here is how to judge your position at the pump:

• Wrong fuel in, engine never started: do not start it. Arrange a drain on the forecourt. This is the cheapest outcome and usually leaves no lasting damage.
• Wrong fuel in, you drove off and the car is still running: stop safely as soon as you can, turn it off and leave it off. Call for recovery or a mobile drain.
• Car has cut out or will not restart: do not keep cranking it. Repeated attempts push more contaminated fuel through the pump and injectors.
• Non compatible petrol car filled with E10: you can drive it. Top up with E5 once you have used around a third of the tank.

After any drain on a diesel, ask the technician or garage whether the fuel filter needs replacing, and watch for rough running, smoke or warning lights over the next few days. If anything feels wrong, get the car checked before small damage turns into a new fuel pump.

Sources

• RAC Drive: Wrong fuel in your car, what to do now
• RAC: Specialist misfuelling patrols
• Allianz Insurance: Misfuelling, what to do if you put the wrong fuel in your car
• Quotezone: Is misfuelling covered by insurance?
• AXA: Wrong fuel cover option policy summary
• GOV.UK: Check if your vehicle can use E10 petrol