Glow plug monitor
WARNING: This page is about a different car, the 2004 Volvo S40, 2003 Volvo S40, 2002 Volvo S40, 2001 Volvo S40, and 2000 Volvo S40. However, it is still accessible from the selected car via links, so may be relevant.
When the key is in positions 2 or 3, the Central electronic module (CEM) controls the brake lights with regards to any problems.
The control light in the combination instrument lights up for 3 seconds and goes off even if the brake is not applied. Should a problem occur, the control lights light up and stay on when the brake is applied.