1990 Audi 100 key won't lock or unlock doors

need to remove the door panel and check the clips that connect to the locking rods possiblr damaged or disconnected

Change the battery in the remote key.

1) Key turns but dose not unlock door. This means that the rod that goes to the back of the lock has came off. This also means that is the reason door is not unlocking with the remote as it pops the lock rod inside the door which pushes the lock open.
2) Key does not turn lock at all but works passengers side door. This means that the locks has been changed in the door, but that doesn't sound like the problem as you say the remote doesn't open the drivers side door so more than likely the rod has came off the back of the lock

Try spraying some liquid graphite into the door lock and keep trying to work the key to turn.

Your door lock actuator have issues

this can be a big problem so will start on easiest first. and try to repair as going, the passenger door the problem with this could be the solenoid for the door lock. and or the switches in the lock. driver door has the same but is the master door. ie if you open door from unlocking at pass door it will only open this door, drivers side will open all doors. flashing hazards this could be because the alarm is not completely turned off this could be because of the doors, mainly the drivers side. try to find the fault with the door lock and get back to me when you have found the fault. you will need a meter to check for volts and the switches, check them as lock unlock from the inside and the key to make sure they working. dpd spray should be able to clean them if you can spray into the lock and housing that is plastic. give a good flush out after its working wipe up the mess from the spray and mear grease on the lock moving parts.

once you fitted new batteries to both fobs you needed to reset the car.....put key in drivers door and unlock/hold key over(windows go down)wait 30 secs then lock/hold over key,(windows go up)wait 30 secs,then press/hold unlock button for 30 secs.you have just reset the system.you have loads of other little gizmos but i will leave you to find them all good hunting....one side parking lights/driveway illumination/and more.
geoff

the car has dead locks via the central locking, it is very common on this car, take the drivers side door card trim off and look to see if the motors are connected correctly.

hope this helps. most likely you will have to change the drivers side motor.

I recently changed the batteries in the key of my 2001 Audi TT. To reprogram it, I needed to take this 1 key to the vehicle, click on the unlock button and within 1 minute, open the driver side door manually with the key. It worked.