S40 Central Locking and Starting Issue

Sounds like a classic case of CEM weirdness induced by a faulty antenna circuit. The bumper antenna on those models is for the remote locking system and radio receiver. Dodgy radio reception lends weight to your antenna theory. The symptoms are all linked via the CEM. Have you checked the wiring and connector to the antenna amplifier in the bumper?

That's helpful. Amplifier, huh? I hadn't thought of that. I haven't torn into it yet. Where exactly is this amplifier located in the bumper? And is there anything specific I should be looking for – corroded pins, loose wires, or anything else?

Based on the symptoms with the error code, needing the key, central locking acting up, and also the radio, that's pointing to the cable or plug connection being defective. You're probably better off taking it to a workshop and getting them to look at it. It's likely to cost around 95€ to fix.

Thanks, much appreciated! I will get the connections properly inspected. Took it to a local shop, and you were spot on with the estimated cost. It was a corroded connector on the antenna amplifier. All sorted now and the car is working as it should again!