VOLVO
CEM-9F21 - Twilight sensor. Short-circuit to ground
System
Central Electronic Module (CEM)
Volvo models this fault code applies to
Applies to the following models:
5 modelsS60 (2001–2009), S80 (1999–2006), V70 (2000–2008), V70 XC / XC70 (2001–2007), XC90 (2003–)
What does this fault code mean?
The twilight sensor gives the central electronic module (CEM) information about the light conditions, which is used to set the brightness of the symbols and the instrument lighting. The fault code is stored when the signal wire from the sensor is shorted to earth.
Possible causes
- Signal wire from the twilight sensor shorted to earth
- The twilight sensor does not work
What you can do
- 1Check that the twilight sensor is firmly fitted and clean and undamaged
- 2With the ignition off, check the wiring and connector to the twilight sensor for chafing, pinching and corrosion
- 3With the ignition off, use a multimeter to check whether the signal wire has contact with earth (the body); if it does, that is the short circuit
- 4Remedy the short circuit if it is found and clear the fault code
- 5If the sensor does not work even though the wiring is intact, it may need replacing; a workshop should do this
Symptoms you may notice1
- The instrument lighting stays on at full brightness all the time
What the fault can affect1
- The control module uses the night brightness setting
General guidance for car owners. Diagnosis on the car can point to a different cause.