If the vehicle HAS NOT been started
The problem when diesel is put into a petrol vehicle is that diesel is much thicker than petrol therefore the lift pump inside the fuel tank will find it harder to pump the diesel through the system.
If the vehicle HAS NOT been started keep the ignition off to avoid causing any damage.
If the vehicle HAS been started
If the vehicle HAS been started it will eventually break down. This is because diesel needs a high compression to ignite, a much higher pressure than a petrol engine can provide. Petrol is ignited by a spark from the spark plugs, when the diesel cannot ignite this causes the engine to stall.
Spark plug removal
In most cases removal of the spark plugs is also needed to clean any carbon build up. Once cleaned, the spark plugs are then reinstalled and the engine to be started.