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on the AA site

Petrol in Diesel

Do not turn on the ignition or start the car
If the car's still under warranty you should check with the franchised dealer regarding their advice and correct remedial action – running with even a small amount of petrol in the tank might invalidate the warranty while some manufacturers advise that seals and filters should be renewed even if the engine has not been run.
Generally a small amount of incorrect fuel should not damage the engine as long as you have not started the car and top up fully with diesel fuel.

• If you've added more than 10% (5 litres in a 50 litre tank) petrol – Drain the tank and refill with diesel
• If you've added less than 10% petrol (5 litres in a 50 litre tank) – Top up with diesel and run normally unless the manufacturer has advised otherwise