Thanks for the very useful information, Russell_S.
In my case, I have a V40 which had a failed LH dipped beam. The light was on if the engine was running and off if the engine stopped (all switches were on). Finally, this was traced to a problem with the relay on the CEM board. When the engine is on, the control voltage on the relay is about 13V, which is high enough to activate the old relay. When off, the battery voltage is about 11.5V, so the relay does not move at all.
The problem is definately with the relay. But to get a replacement relay is not easy. Dealers do not have the relay as a component but would charge about £450 for the whole CEM. In the end, I found a replacement from RS (stock number 217-2876) and only costs £1.47 plus VAT and del.
Link here:
http://uk.rs-online.com/web/search/s...7-2876&x=0&y=0
I have also attached the datasheet here. It is 15A compared with the old 20A, but well enough for the dipped beam (about 5A).
After replaced with the new relay, my V40 returns to normal.