It comes on occassionally if I cruise at over 85mph for a period of time. The dealers says it is just an occassional mis-fire. The light stays on for approx 5 days before reseting - can I manually switch it off ?

 

On some models if you start and stop the car 4 or 5 times the light goes off.

You can do a manual ecu reset but depends what year your car is, classics are easier.

 

It's a WRX MY02

 

TBH, I think you should post asking for alternative dealers in your area! It's a warning light meaning something is wrong and needs checking. I wouldn't let him fob you off with it's an occasional mis-fire. These cars have some quirks but this aint one of them!

 

Even if the CEL has gone off the ECU should store the fault in its history, so the dealer can read it.