Starting from 2000 (S80 from 1999), Volvo P2 platform cars no longer use a traditional mechanical light switch to control headlights. Instead, they use a CEM (Central Electronic Module). The light switch only sends a request via the CAN bus to the CEM unit, which activates the lights through internal relays.

Intermittent lights: Sometimes only one low/high beam lights up, sometimes both are dark. Heat sensitivity: Lights may go out as the car (and CEM unit) warms up from outside temperature. Random behavior: Tapping in the driver's footwell may briefly turn on the lights. Bulbs intact: Fault cannot be fixed by replacing bulbs or fuses.

The CEM is located in the driver's footwell, under the dashboard (behind the knee pad). It is a large black plastic box with multiple connectors and houses some of the car's fuses.

A new CEM unit costs about 500–800 € from Volvo plus programming labor, but often the fault is in the small relays inside the unit that wear out over time. Relay replacement: The CEM unit contains small relays soldered to the PCB or press-fit to a socket (depending on the exact year). Separate relays control the low and high beams (right/left separately). Solder joints: Often the relay pins break from the PCB (cold solder). Reflowing the joints with quality solder fixes the problem permanently. Rear wiper: Odd rear wiper behavior (getting stuck or wiping by itself) also often stems from the same CEM unit and a jammed relay.

Battery disconnection: Always disconnect the negative battery terminal for at least 5 minutes before removing the CEM unit to avoid triggering fault codes in the airbag system or burning sensitive circuits. Programming: If you buy a used CEM unit from a salvage car, it won't work directly because it's coded to the previous car's immobilizer. Using a used unit requires software transfer (cloning) from the old to the new unit at a specialist shop (e.g., Sercap or similar services). Ventilation: In some early models, the CEM unit runs very hot. Make sure the heater vents don't blow directly on the unit if possible.

If low beams don't work, try flashing the high beams. If high beams work normally but low beams are dark (and bulbs/fuses are OK), the fault is 90% certainly in the CEM unit's relays.