I have a fault code along with the service required light (car with spanner) on my 54 plate Astra 1.7CDTi. Does anyone know which sensor, C refers to.

The light sometimes goes out and it is possible to clear the code but it comes back frequently and puts the car in limited operating mode. The fault even reappears when the engine isn't running and the light can go on and off while driving which makes me think it is a wiring or connector issue but I'm not sure where to start.

Any advice would be gratefully received.