I recently had this problem and thought I would tell you how I resolved it so if you run into this,well, you know. I have a 2002 C4 Cab with tiptronic trans and recently, everytime I would try to use the keyfob to lock the car, I would get an annoying double horn beep and the door would not lock. Same behavior with every keyfob.

Other interesting symptoms included:

-Interior locking button not working
-Interior rear defog button that is under the interior locking button not working
-Interior lights not working
-Power window buttons not working, but windows still working when putting the top up or down

Durametric read the alarm codes and I got code 34 and 60. Would clear the codes and then try and lock the car again, get the double beep and then the codes would appear again.

I suspected a fuse since the interior lights were not working. I check the Bentley Manual for the interior light fuse position and found it at C3--15 amp.

It also listed for that fuse, Alarm Control Module, Convertible Top Control Module, Inside Light, Power Windows, Sunroof.

Hmmm. Checked it and the fuse was blown. Replaced it and all is good now.

Recheck the alarm codes after erasing them and they don't come back now.

Hope this helps someone in the future. Take care!

 

Huh. Mine did this after my battery was dead for a while. I just put my key in the drivers door and turned it to lock then unlock and it solved it for me.

 

That didn't work for me. I'm not sure if you read my post but I had a blown fuse and there is no way using your key to lock and unlock manually is going to fix a blown fuse.