(T) Engine Management query!

Why is it when a Coil goes on the Stilo and you get the engine management and engine fault warning displayed on your dash, if you replace the faulty coil and restaert your car (3 times) the fault goes and the engine light goes off.

However when I recently had a MAP sensor fault that wasn't a fault as car ran fine (contacts cleaned) the car only displayed engine fault on startup not the usual every 2 minutes but the engine fault light remained on even though the problem had gone even after 3 start ups. It took the AA vixon diagnostics tool to clear the fault.

So my question is why can you sometimes clear some faults by repairing the fault and remove the Engine light but not other faults and have to have fault cleared by professionals ie The AA or FIAT?

Depends on the error.