My s model won't lock. I left the key in it and the driver door open for about 15 minutes while it dinged the whole time. I came back and closed the door and tried to lock it but it keeps unlocking itself immediately. I does that with the key, the remote and when trying to lock it inside the vehicle / running or not.

 

Faulty door lock / microswitch. Probably the most used one ie. driver's.

 

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Haviing experienced the failed door lock microswitch several times, I agree with JagV8's suggestion.

If it is a failed door microswitch you would find that the interior lights stay on, and you would see a message on the dash "drivers door open" or maybe another one of the doors.

But it could also be the boot/trunk or hood/bonnet switch, this would not cause the interior lights to be on, but would stop the car from locking. Likewise you should also see a message stating that one of these are still open.

 

Door lock issues have been covered in the forum. You can find lots of info by doing a search in the S-Type forum using the search or advanced search feature.

My '01 S-Type also experienced the lock problem - they locked, then immediately unlocked. No interior lights came on, no dash messages, etc. Just seemed that the locking linkage in the driver door was fatigued and not strong enough to complete the task of locking the door. And when that fails, the locking mechanism logic circuit kicks in and unlocks all doors as a safety precaution. Unfortunately, I can't recall our workaround but we did eventually resolve the problem thru trial and error.