Most people have been in a situation where the garage is not 100% sure on which part has failed so decided to load the parts cannon and basically just keep firing new parts at it until it fixes it. This is bad practice and could cost you £1000’s!
This process isn’t the correct way to fix a fault. When the ECU logs a fault code and brings the engine management light on it would have stored a fault code within the ECU. This can be integrated with the correct diagnostic hardware this would then tell you where you need to start.
It is possible to test individual components on the vehicle with the diagnostic equipment enabling the tester to see if they are responding as they should be.