Safrane Central Locking Failure

I had a similar issue on my old Opel Astra, although the symptoms were slightly different. The central locking system went haywire, and it turned out to be a faulty door switch. Have you checked the wiring harness where it passes from the car body to the door? They are prone to damage from repeated opening and closing.

Wiring harness, huh? I'll definitely give that a look. But you mentioned 'door switch' – is that the same thing as the door contact sensor? And where exactly should I be poking around with my multimeter?

Yes, by door switch I mean the door contact sensor. In my experience, and given your car's age, I would strongly suspect that the connector is affected by corrosion. As the issue affects all doors, however, it is more likely that the root cause sits in the module that reads the door sensor signals and controls the automatic locks. I would recommend having a qualified workshop check the module and the associated wiring, because troubleshooting it yourself might get quite complex.

Okay, thanks for the advice! I took it to the shop, and you were right. Turns out a cable for the door contact signal was defective; the mechanic said corrosion had eaten right through it, just like you suspected. 95€ and a new cable later, the car's back to normal. Seriously appreciate the help!