What happens
A unit cannot communicate because its required supply or internal control power is absent.
Typical symptoms
Several heads or one circuit report a missing partner; the affected controller shows no normal status.
Checks that distinguish this cause
Determine which units and accessory boxes have independent supplies. Trace actual power before testing bus signals, and compare the affected unit with the documented normal standby state.
What can look similar
The whole system may be inhibited by a safety or configuration error despite available supplies.
Repair direction
Restore the confirmed supply defect while preserving protection and grounding.
Verify the repair
Confirm every required node powers up and the communication fault clears through the OEM sequence.
Equipment-specific limits
Some interconnect terminals carry line voltage or line-referenced signals. Wire count, terminal numbers, polarity, shielding and permitted splices are model-specific; do not treat a bus as R/C/Y wiring.
Practice this diagnosis
This cause is a reference topic; it does not yet have a dedicated simulated call. Browse the practice board.
References and applicability
Original HVACSim explanations with manufacturer and instrument-maker references for further reading. These resources support the subject; their model-specific examples are not specifications for every system.
- GREE: GREE communication-fault diagnostic guidanceA GREE-specific example of wiring and board fault isolation; E6 is not a universal code.
- Haier: Arctic Next Gen service manualAW/1U EH2VHD model examples of inverter, motor, sensor and circuit diagnostics; obtain the installed model manual.
Editorial review: 2026-09-29. Verify the current manual for the installed model.