I beleive it's 126 542 01 32. I will double check when I get home. This part seems to cause a host of issues from lights not coming on to lights not shutting off as well, but appears to also cause the bulb out indicator problem. There are very few posts that actually list this part or it's proper name "Lamp Control Module". The pricing also seems to vary widely with MB South Atlanta listing it at $275, NLA at AutohausAZ, and $58-$70 at Peach Parts. Depending on your skill level as a DIYer, you could just resolder parts of the board (the issue seems to be cracked solder joints or board traces).
Have you verified that ALL bulbs are the same brand and wattage left to right? in the rear. ALL bulbs must have silver bases not brass. Have you confirmed both license lights work as well as the high mounted 3rd brake light.
I tried resoldering and frankly it is NOT a cureall for the W126 electronic modules ...
I have not changed bulbs in over one year so why would they suddenly change if they still work?
License plate light are OK and evenly lit.
Before replacing the module, I will clean out all receptacles and bulb contact points but getting a bulb light ON without turning anything on suggests a defective module... apparently it is mostly made up of current feeds to a single integrated circuit. If I DO replace the module, I open the old unit and will find out WHAT the IC is ...
Wasn't trying to bill resoldering as a cureall, but it is always an option. Good call on cleaning everything up before replacing the module, but what you are describing does sound like a module or harness fault. So make sure you clean up the chassis grounds for the headlight harness too. Best of luck to you on getting this sorted.
Check all the bulbs for a dark grey color on the bulb glass. They all should be bright and clear. A darkened bulb glass means the resistance of the bulb filament has increased its resistance hence the bulb fault light is turned on even though the bulb will light up! Again, are you sure you're not stepping on the brake pedal? when you start the car. Are the headlight bulbs the same brand and wattage?
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