VW Golf MK3 - Central Locking problem

[Guest93]

Basically I own a MK3 Golf...potentially not for much longer as it probably going to get stolen. Here's the problem...

I lock the Car in the usual manner only to discover on my return it has unlocked itself again.

I believe its something to do with the vacuum pump ?

Any suggestions how to fix

[torbjorn]

A common fault is broken wires in the wiring loom to the driver's door, it is easy to pull off the rubber boot to check status of the wiring.
Another fault is that the switches that sense lock/unlock operation in the door lock get clogged by dirt and old, stiff grease. In winter, that switch problem may get even worse due to icing.
It is fairly common that owners of Golf mk3 use to disassemble the door locks for cleaning and lubrication every year.