Cause
Faulty air conditioning refrigerant expansion valve
The electronically actuated valve that meters refrigerant into the evaporator is not reaching the opening it is commanded to.
Note what this is not. It is not the distribution valve, which decides which evaporator gets refrigerant on a vehicle with more than one — this is the valve that sets how much gets through, and every system has one. It is also not the classic mechanical expansion valve, which has no wiring and cannot be at fault in this sense at all; only a motorised one can be commanded, and only a commanded one can disagree with the command.
The failure is usually mechanical even though it is reported through a circuit. A stepper motor loses steps, debris or migrated compressor oil stiffens the pintle, or a learned position drifts. The wiring tests fine throughout, which is why an electrical diagnosis on its own tends to clear this fault as a fault in nothing.
Its signature is weak cooling rather than none, varying with demand — and on a heat-pump vehicle, poor cabin heating from the same part. A system genuinely low on refrigerant cools badly everywhere and consistently, so a complaint that changes with conditions is evidence against a regas rather than for one.
Typically specialist work. Needs equipment or training most home workshops do not have.
Codes this can cause (1)
Codes where this is a likely explanation first, rather than merely a possible one.
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