Air-source heat pump outdoor unit beside a house in the snow

Generic photo: PeterEastern · CC BY-SA 4.0

Communicating system (Carrier Infinity / Bryant Evolution) shows 'unit not found' or lost communication

Communicating systems use a four-wire bus: A and B carry data, C and D carry 24 VAC. Carrier's trade guidance says most faults come from the connecting wiring, so isolate devices one at a time before replacing boards.

Cross-checked ~60 min moderateBrands: Carrier, Bryant

Before you start

Indoor and outdoor units have line voltage: CAUTION
Power off before moving bus wires
Refrigerant faults listed in the event history are BLOCKED: refer

Diagnostic steps

Unlock all 7 steps with expected values

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OnCall Tech is a reference tool for trained technicians and can make mistakes. Always disconnect power and confirm with the manufacturer's documentation before you work.

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