My 1998 XJ8 is not locking the rear driver's side door... Would this be the door module, or the latch actuator?

If I hit the lock button on the key fob or the central locking switch, it will lock all doors, except for the rear driver's door. If you open the door that won't lock, then the security system will sound off.

I have to believe that it is the lock actuator. Someone give me their take on it.

Thanks for looking.

 

Try a hard reset and see if it squares away the security ECU back to zero state

 

+1 for Lady Penelope's suggestion, if that doesn't work, you may have to go to the control module itself in the drivers door and disconnect it for a few minutes and reconnect it. But it could be that actuator as you suspect, thus pulling the door card and metering out the connector to it.

 

I have a 1998 V8 Vanden Plas and have a problem with the passenger's rear door lock. I noticed your answer and will try these to correct the problem Many thanks.