DS4 Hybrid AC Failure

Sounds like classic AC compressor trouble. Is the compressor clutch engaging at all? Have you checked the refrigerant pressure? Low pressure will prevent the compressor from kicking on. Also, any unusual noises coming from the compressor area?

The clutch isn't engaging. I don't have the equipment to check the refrigerant pressure. No unusual noises from the compressor area. Could this be an electrical fault affecting the control unit?

Based on your description, especially the lack of clutch engagement and no unusual noises, a defective air conditioning control unit is a strong possibility. While DIY diagnosis can be helpful up to a point, given the complexity of modern HVAC systems and the potential for refrigerant leaks, I'd recommend taking it to a reputable workshop equipped to properly diagnose and repair your vehicle's AC system. Avoid further use until repaired, as that could exacerbate the problem.

Took it to a trusted workshop and they agreed: the air conditioning control unit was defective, causing the compressor failure. Control unit replacement and system recharge cost 980€. The system is now working.