Rear right door won't lock

[Gary44]

I have an audi a6 2013 and the central locking isn't locking the rear right door. I've tried child locks, new battery in key, manually locking. Nothing works. I'm guessing it's the lock/actuator but how can I be sure before ordering part online? Thanks in advance

[Steve Q]

I'd recommend a diagnostic check to see if any fault codes flag up. It could be the actuator or module