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My tailgate is not releasing on the central locking key so between showers have been poking around trying to find the issue.

When the tailgate is open the dash warning lamp illuminates.

Central locking works OK on the doors, but have to use the key for the tailgate.

Does the actuator look OK? I thought the plunger/arm looks like it is too far from the lock mechanism.
Is it possible to apply 12V to the red/blue and brown wires to check if it moves? Which wire is which?
Is there any test I can do with a meter on the central locking wires (brown/brown with yellow band)?
Incidentally, I replaced the lock mechanism itself with a part from Germany last year.

00896 - Trunk Lock Unit (F256)
27-10 - Implausible Signal - Intermittent

Just back-probed the power supply to the actuator. Ground wire seems fine. No 12V to the red wire on activation of the central locking (by key). Other doors are locking and unlocking OK. Have to use key to unlock tailgate.

Would the voltage be applied only on operation of the tailgate handle or when central locking is locked/unlocked??

Any ideas on where the problem might be?

Yes, I’d expect the unlock and/or release signals to be short pulses.

Back probing normally only works well if you do that with all connectors still in place, unplugging connectors and checking what is on pins tends not to give good results.