Misfuelling: What to Do When You Fill Your Car with the Wrong Fuel

Updated 25 March 2025 | Published 22 February 2024

Ever pulled up to a petrol station, grabbed the pump, and filled up—only to realise moments later you’ve used the wrong fuel? Misfuelling happens more often than you’d think, and the most common mistake is putting petrol into a diesel car.

Why? Diesel pump nozzles are wider than petrol ones, making it harder—but not impossible—to insert them into a petrol car’s filler neck. However, petrol nozzles fit easily into diesel tanks, leading to more frequent mix-ups. If you act quickly, you can minimise the damage.

Petrol vs Diesel: Why It Matters

Petrol and diesel engines work differently, so using the wrong fuel can cause serious problems:

Petrol in a Diesel Car – Diesel engines rely on fuel for lubrication. Petrol acts as a solvent, stripping away lubrication, damaging injectors and pumps, and potentially seizing the engine.
Diesel in a Petrol Car – Diesel is thicker and burns differently. A petrol engine can’t ignite it properly, leading to misfiring, excessive smoke, and damage over time.

Symptoms of Misfuelling

If you’ve accidentally put the wrong fuel in your car, watch out for these warning signs:

Petrol in a Diesel Car:

Difficulty Starting – The engine may struggle or fail to start.
Noisy Engine – A rattling or knocking sound signals trouble.
Loss of Power – Acceleration feels sluggish and unresponsive.
Smoky Exhaust – Black smoke from the exhaust indicates unburnt petrol.

Diesel in a Petrol Car:

Knocking Sounds – Diesel ignites poorly in petrol engines.
Poor Acceleration – The car feels slow and hesitant.
Engine Warning Light – A common misfuelling indicator.

What to Do If You Misfuel

1. Don’t Start the Engine!

Even turning the key partially can pump contaminated fuel through your system. Leave the engine off and seek assistance.

2. Move Your Car to a Safe Spot

If you’re still at the fuel station, push your vehicle away from the pumps to avoid blocking others.

3. Call for Help

Contact your breakdown cover provider or a specialist fuel drain service immediately. They can usually drain the fuel system roadside.

4. Drain and Refill the System

For diesel cars – The tank, fuel lines, and filter must be drained and refilled.
For petrol cars – A full fuel flush may be required to remove diesel contamination.

5. Inform Your Insurer

Let your insurer know in case any repairs are needed. Some policies cover misfuelling incidents.

How to Prevent Misfuelling

Double-check before filling up – Get into the habit of confirming your fuel type.
Look at the pump labels – Most stations use colour coding (black for diesel, green for petrol in the UK).
Use a misfuelling prevention device – Some modern cars have built-in safeguards, or you can buy an adapter.
Place a reminder near your fuel cap – A simple sticker can prevent an expensive mistake.

Final Thought

A moment’s distraction can lead to misfuelling, but quick action can save your car from costly damage. Stay aware, act fast, and keep your engine safe.