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A high-voltage air conditioning compressor has been serviced with the wrong lubricant — almost always the PAG oil used in every belt-driven system.

This is a safety defect rather than a reliability one, and it is worth stating plainly because the mistake is easy, invisible and repeated. A conventional compressor is an earthed lump of metal spun by a belt; its oil only has to lubricate. A high-voltage compressor has a motor winding sitting in the refrigerant stream at several hundred volts, so its oil is also an insulator, and that is why these systems specify a non-conductive polyolester oil.

PAG oil conducts, and it is hygroscopic — it absorbs water from the air, which conducts far better. Introduced into a high-voltage compressor it degrades the winding insulation, and because the refrigerant circulates through metal pipework running the length of the car, a breakdown can put voltage onto components nobody expects to be live. The vehicle's isolation monitoring is what stands between that and a person touching it.

Contamination does not undo. The oil is carried everywhere the refrigerant goes, so the remedy is a full system decontamination and usually a new compressor — which is why the question to ask before any air conditioning work on an electric or hybrid vehicle is whether the recovery equipment has ever handled PAG oil. A machine shared with conventional cars is enough to transfer it, and a workshop that declines the job for that reason is being careful rather than unhelpful.