Mercedes-Benz W124 light failure warning comes on
W124 light failure warning is on: where do I look for the fault?
Note when the light comes on. If it comes on when the parking lights or headlights are switched on, the fault is in the parking or licence plate lights. If it comes on already when starting, the fault may be in the brake light. The monitoring unit watches the current draw of every bulb, so even one wrong bulb turns the light on.
Which bulbs suit the W124 small lights?
Front parking light is T4W (BA9s base), rear parking light R10W, side indicator W5W and licence plate light C5W. Both sides must have bulbs of the same wattage. A 5 W bulb fitted in the rear parking light instead of a 10 W one often turns the warning light on, as does a 24 V bulb in a 12 V system.
The warning light is on even though the bulbs are fine: what should I check?
In old cars the bulb holders and connections oxidise; clean them. The licence plate light wire can be broken at the tailgate hinge. The light monitor box is in the engine bay on the driver's side near the fuse box, and the solder joints on its circuit board can be weak, so they are worth checking.