VOLVO
CEM-U210929 - Back-up battery. General Signal Failures. Signal signal invalid
System
Central Electronic Module (CEM)
Volvo models this fault code applies to
Applies to the following models:
2 modelsV40 (2013–), V40 Cross Country (2013–)
Show models by year and engine (2)
What does this fault code mean?
On the V40, the central electronic module (CEM) monitors the voltage from the back-up battery, a secondary battery. The fault is stored at ignition position II when the CEM receives a signal about the back-up battery that is not valid. The automatic engine start and stop function is then switched off.
Possible causes
- Open circuit between the back-up battery and the engine compartment fuse box
- Damaged fuse for the back-up battery
- One of the two engine-bay fuses is damaged
What you can do
- 1Check the fuses in the engine compartment, especially the two that serve the back-up battery, and replace a blown fuse with a new one of the same rating
- 2Check that the connectors at the back-up battery and in the fuse box are tight and free from oxidation
- 3Use a multimeter to check that the cable from the back-up battery to the fuse box has no break
- 4If you have a diagnostic tool, follow the back-up battery voltage in live data and check that it sits close to the car's other battery voltage
- 5If the fault returns although the fuses and connectors are sound, have a workshop check the supply to the back-up battery
Symptoms you may notice2
- The automatic engine start and stop function does not work while driving
- The automatic engine start and stop function does not work at idle
What the fault can affect1
- The automatic engine start and stop function is switched off
General guidance for car owners. Diagnosis on the car can point to a different cause.