I have two fob keys but one is only able to unlock/lock doors using the button, when i try to start the car it shows the mobilizer light. How can I reprogram the key? I have one key that works fine.
I have a 2004 Audi A4 1.8

 

There are some reports of keys being successfully coded using vagcom but you need some special importer/workshop codes. I'm not sure of the exact procedure.

the only sure way is to take it to a stealership. bring your own lube.

 

Somewhere on this forum I found directions for reprogramming the key fob. It worked! Neither of my remote keys worked, even with new batteries. Only the red light blinked. Do this: 1) put one key in outside door lock 2) put other key in ignition and turn on without starting engine 3) reach through window to turn door key to locked position 4) at the same time press the unlock botton on the ignition key for several seconds. Stop. Press again. 5) Remove key and test.

I suspect that they got unprogrammed when we switched out the dead car battery -- or the car sat for 4 months.
Thanks for the help Audi Forum!