V70 Door lock problems

A friend has a 2002 V70 cross-country. MOST of the time, when he attempts to lock his driver side door, the door locks and immediately
unlocks. This happens only on the driver door all other doors simply lock and stay locked. Using the key fob, the mechnical door lock with a key or even the inside switch has the same result.
I say most of time since after doing it several times, you can get it to stop this and simply lock all doors. And there are times when all is fine doors lock without any problems.
Any ideas?
Thanks.
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One of the 4 door lock motor/latch/switch assemblies have failed. The dealer needs to connect to car to Vadis to determine which door lock switch has failed. The door lock/latch assembly is all one piece. There is a door opened/closed switch and a lock/unlocked switch inside of each of the 4 door latch assemblies. The lock/unlock switch has failed in one of the 4 doors. The DDM (Drivers Door Module) monitors all of the 4 doors activities and settings. When the DDM sees that one of the door lock/unlock signals is still in the unlock mode when it should be in the lock mode a message is sent to unlock the drivers door. Sometimes the offending door lock knob will not activate and this will be the faulty latch assembly. Mostly the door lock button will lock but the lock/unlock signal for that door will be kept as unlocked because the lock signal was never sent due to a faulty lock/unlock switch. The DDM remembers the last signal received was for unlocked. In Vadis you can monitor each Door open/closed or each lock/unlocked switch. Of course the dorrs have to act up to see the faulty latch assembly while being monitored in Vadis.
This may not work for your problem. There is a way to reset the door locks and sunroof if they fail to operate. This includes a non working lock button on the drivers door panel. This is for all S80's, 2001 up S60 and V70's. Key on, dash lights lit. Close both front windows. Hold both front window switches, in drivers door, in the up position for about 6 seconds or so, release. Remove key. Close all doors, hood and trunk lid. Using the remote, lock and unlock the car twice. The UEM, Upper Electronic Module is now reset. The above will not correct a faulty part.
BD ---------------------------------------------

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Thank you very much for your detailed and informative response. I will try to reset the module see what happens. I appreciate the help. Problem here in MA is that anytime you show up at any Volvo dealer, they want $800 min. no matter what they do.
Fred

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