I am having problems with the rear offside door lock on our 2018 Super 2.0L SE Technology diesel estate. The drivers door switch panel includes the two child lock buttons. When locking the car the rear offside door remains unlocked, therefore we have to keep it locked via the small turn screw on the edge of the door. Having looked into new lock prices, I decided to try a second hand one from a 'reputable' supplier, only to find that this replacement is doing the same thing. I conclude therefore that either I'm unlucky and the second hand unit is faulty too or there's a problem with the wiring. Does anyone have access to wiring diagrams for the locking system, or any suggestions as to how to test the locks? Thanks.

I would want a known working lock before diving into wiring diagrams. If nothing obvious looks broken in the connectors, then I’d firstly make sure the used lock was compatible with the rear electronic activation child lock function. If it’s not, the lock will not work.

I think a brand new rear lock genuine from TPS for mine a few years ago was £250, with the drivers door switchable child lock function.

Thanks. It was that sort of price and being on a pension only that led me down the second hand route. I couldn’t justify the cost if it wasn’t a faulty lock causing the problem. The part numbers are identical right down to the suffix so it should be compatible in regard to the child lock function.

Since posting, I have discovered a test procedure for checking the lock out of the car, so I’ll give that a go on the original. It’s an A.I script so could be wrong, but I have nothing to lose by trying.

I can probably find wiring info for you, but will need VIN. I'll message you privately for you to give me that.

On a lot of models, the rear door modules are LIN sub-modules of the front door modules on same side. This has a single-wire data connection between front and rear.

This wire may have broken at the point where the driver's door loom flexes, rather than where rear door loom does.

Have you insected for wiring damage at either of these points?

Edited 3 July3 Jul by Breezy_Pete

Thanks, I haven’t yet inspected the looms and wouldn’t have even thought about the drivers door, so great call. I shall try to do that later today and report back.

Some versions seem to have rear door locks wired direct back to BCM, others via door modules. If rear windows are electric and are working, it's probably not the LIN wire.

Ah, ok. Windows are all working as they should.

What is puzzling me is that when I first installed this used lock earlier this week, the door locked as it should, several times. It is only now a couple of days later that the auto lock when driving has started clicking and the door now doesn’t lock, just as the original. There is an audible clunk from the door when locking but it doesn’t actually lock.

So following on from the help and suggestions from Breezy_Pete, I have tested my old lock on the bench and inspected the wiring in the car. It appears to be a common problem with these locks that the microswitches (4 in total) play up and this seems to be the case with mine. I was getting some very spurios resistance readings from some of the switches, therefore I shall be ordering a brand new lock. I was obviously unlucky with the second hand lock as it appears to have the same issues, so that's a lesson learned. Many thanks to Pete once again for his help.