Chosen Solution
Have a keyfob that’s not working. The ‘open door’ does not work. Close door and trunk buttons work. Compared it to spare fob, and noticed two instances where tiny copper/gold plate circles were missing on the pcb board. Tested for continuity and failed. Good fob had continuity. Wondering if carefully adding a drop of conductive glue in both areas would be a fix. Photo has two arrows noting areas in question. The circled area represents the plate cover that exists on those two areas, on the good fob…(which are missing here)
Answering for completion This keyfob is likely not repairable without loss of functionality, and conductive glue will not be enough. In this case the damage is quite deep and isn’t just minor surface level damage. You’ll likely need a new key. Since you have a spare keyfob that’s 100% intact, you can often use that even on a Mercedes. If you didn’t have that you’d need to have a key made at a locksmith who is authorized by Mercedes to program a replacement. You’ll need to know the model and year (chassis code helps too - think W204, for example) to find the steps to do this, as we do not know. The W126 is too old for a keyfob - this is at least a W2, C2 or R2 keyfob. For example this was used from the W202-W205, and various other models.
