PJmak

I have thee keys for my c55. Batteries are not the issue in any of them so lets get out of the way first. One key is the new style chrome key while the other are older style black keys, one of which looks like its never been used. I bought the car in 2016 and have been using the chrome key and its always been fine up until recently when it started having a difficult time locking and unlocking the car. Never had issues starting the car. I dug up one of the spare keys and it worked fine for everything until recently also when it wouldnt lock and unlock but it always started the car. I dug up the third key and that one only starts the car but also cant lock and u lock. I scanned the car and figured I had a bad alarm siren which was popping the fuse in the trunk. Disconnecting the siren resolved the locking problem but only for one of the black spare keys so now I have three keys but only one has full functionality. what gives?

Robjr

PJmak.....

This sounds like a tuffy.....I hate to say this but a visit to a dealer may be in store to reprogram the key.......wait for forum good guys to respond before your dealer visit...

By the way, pm me your Instagram w203 car forum so I can visit.....

Thanks.....

Rob

JonMBZ

Try pointing the key are the infrared sensor on the door, if it unlocks, the it's probably the antenna in the back window. The key can unlock via radio frequency and IR. I have 3 keys and none have this issue, but I know there were antenna issues back in the day.