I cannot lock my drivers door with a key. I can lock it manually from the inside using the associated locking knob. I can lock both the drivers and passenger doors simultaneously using the key on the passenger side when both doors are closed.. I also can lock both doors from the inside with the dash central locking button. What is my issue with using the key on the driver's side key lock?

Either the lock has seized or it's been changed and your key doesn't fit correctly. Doesn't take long to whip it out and take a look at it, only 1 bolt holding it in.

nluk100 wrote: I don't believe the lock is seized because it operates to open and close using the passenger side key lock, driver's manual turn knob and the dash console switch. It is just when I use the driver's side key lock that it doesn't lock. Your suggestion of a changed lock might be the answer.

I didn't mean the door locking mechanism, I meant the actual door lock (the bit the key goes it). As I said, it's easy to pop out and check it. Plus, you can also attempt to re-key it to your existing (with a file and some patience) ... www.928.org.uk/56-re-keying-porsche-928-locks.html

If you've got the additional cable on the bottom of the lock (I guess you will as it's an 89 S4) then you'll need to file the levers in this bit too.