What happens
An inaccurate or missing input prevents the board from recognizing the proper operating state.
Typical symptoms
Unexpected defrost timing, fault codes or persistent outdoor ice.
Checks that distinguish this cause
Inspect isolated connections and compare sensor behavior with the model temperature/resistance data or specified digital procedure.
What can look similar
Normal frost during heating is different from ice that remains after the expected defrost sequence.
Repair direction
Repair the proven input connection or replace the approved sensor.
Verify the repair
Confirm input plausibility and the prescribed defrost operation.
Equipment-specific limits
Timing, sensors and test modes are specific to the board. Never bypass safeties or apply arbitrary jumpers to force defrost.
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.
- Resideo: RTH6360 thermostat installation and setupExamples of heat-pump O/B and staging setup. Menu numbers vary by thermostat.
- Copeland: CoreSense diagnostics for scroll air-conditioning compressors (AE8-1379)Model-specific diagnostic examples; flash codes are not universal.
Editorial review: 2026-09-29. Verify the current manual for the installed model.