Renault Master AC Undervoltage

The symptoms you describe, coupled with the undervoltage error code, do indeed point towards a potential issue within the A/C system's electrical supply. While a faulty relay switch is a plausible cause, the undervoltage could stem from other components as well. Have you performed a thorough inspection of the wiring harness leading to the A/C compressor and the HVAC system's control module? Furthermore, what is the measured voltage at the A/C compressor's power supply connector when the system is commanded to engage? Is the A/C system pressure switch functioning correctly and reporting valid pressure values?

Voltage at the compressor... good point. I'll have to check that directly. As for the wiring harness, visually it looks okay, but I haven't load-tested it. The A/C system pressure switch, how can I properly test its function? Just measure resistance, or is there a more involved procedure?

Testing the A/C system pressure switch typically involves more than a simple resistance measurement. Consult the Renault Master's official repair manual for the precise testing procedure, as it often involves monitoring the switch's output voltage or signal frequency as the system pressure varies. Given the complexity and potential for refrigerant release, I would strongly suggest consulting a qualified technician equipped with the appropriate diagnostic tools and safety equipment. Undervoltage issues can be notoriously difficult to pinpoint without specialized equipment. You may be facing a faulty relay switch but it's worth going to a workshop to get it sorted out.

Thanks for the advice! I was hoping it'd be a simple fix, but undervoltage gremlins sound like more than I want to tackle myself. I took it to a local shop; they confirmed it was a faulty relay, likely damaged from a spike. They replaced it, total cost 240€, and now I'm back to cool air. Appreciate the help!