Amrs

Hello, I have W204 2009, suddenly one key working perfect and the other one not detecting very much!! I mean need to be very close from the car to lock the door and turn car ON.

I changed the battery but still the same

Please for your help and advice.

Thank You

Racin_fool

If you have 1 key working normal and the other key is not working normally, then you already diagnosed it for yourself. More than likely something is wrong with the key that is not working normally. You'll have to visit the dealership to order a new key for the vehicle if this issue concerns you.

eXstasy

Are you sure the new battery you put in has full charge? I say this because I bought a pack of batteries on Amazon and they were almost dead. If you don't want to take a volt meter to the batteries then you may want to swap the batteries between the good key and bad key just to be sure.

GRP

I have found my key remotes fussy re batteries - a brand new premium brand battery will not work yet the next one in line in the blister pack will work fine. Both batteries will be good on the volt meter.

Swap the batteries over between remotes.

I keep my keys in a"faraday" box as my front hall table is cloe enough to the C63 to communicate.

Steiner

These key fobs eat batteries faster than any other car I've owned. It's not a huge deal but I'll start randomly getting "key not detected" messages on the dash about 2-3 weeks after replacing the battery. I can usually get another 1-2 weeks out of the battery at that point. I've tried all different brands of batteries and it's the same thing.