Does anyone know how to reset the engine management light on my 2001 Mondeo Mk3?

The petrol cap was loose and I have been told this might have been the cause for it to come on. I have tried disconnecting the battery and its still on.

The dash code says it has no fault. It says dtc-none. another code is on the digital dash dt684E.

Anyone any ideas? it drives ok

Thanks!!!!