C6 glow control unit failure

Sounds frustrating. Does the engine struggle to start, especially when it's cold? What color is the smoke? Has the car started driving differently?

Engine runs rough, like a misfire, and stalls when cold. Smoke is greyish. I recently installed a new battery; could that be related? Voltage was good before installation.

Grey smoke and rough running points to glow plug or fuel issue. Since the light is on and you're finding misfires and grey smoke, the glow control unit might have an issue, like a defective cable or plug connection. Get it checked at a workshop.

Took it to a local workshop. The glow control unit was defective due to a corroded connector. Total repair cost was 95€. Everything is working fine now.