[Symes]

Sounds like you might well have found the culprit!

Next thing is to get the wiring diagram and see where the signals are coming from, most likely the ECU on the top of the air filter box... the ECU has to be removed to change the air filter(!) So its possible that the connector contacts might need a clean or even have been bent or broken.

At least when you have the wiring diagram you can do a continuity test and rule in/ out a break or short in the cables.

I think this thread demonstrates (again) the power of the Forum; people willing to offering advice, their own experiences, equipment and time in order to help each other out.