I had the exhaust off my car to sort out a weeping diff. It a standard exhaust so therefore had to come off where it connects to the manifolds (headers).
All 4 O2 sensors were disconnected but all replaced when the job was compelted. when i fired up the car the check engine light was on but the car ran perfectly as it did before. I've taken it to the dealer to have it reset (i guessed it had to learn parameters) and this solved the problem-they confirmed that no faults were present or stored.
However, when i switched the car off again and then restarted it about an hour later the light was on again!!
Does anyone know what has happened or how i can fix it??