What happens
The intended pilot command does not reach or actuate the solenoid.
Typical symptoms
Heat pump stays in the wrong mode even though the compressor runs.
Checks that distinguish this cause
Identify expected O/B logic and current mode, then verify the command across the actual coil terminals. Separate thermostat setup, cable, board output and isolated coil tests.
What can look similar
A main valve can stick even with a valid electrical command. Defrost may intentionally change the command.
Repair direction
Repair the proven command path or replace the approved failed solenoid.
Verify the repair
Verify heating, cooling and the documented defrost behavior.
Equipment-specific limits
Electrical command does not prove the main valve shifted. O/B energization depends on the system design, and some conditions are normal during defrost.
Practice this diagnosis
These simulated calls exercise this component or repair family. The article describes a broader diagnostic process; a call may represent a different cause within that family.
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.