What happens
The internal moving assembly does not route refrigerant as intended despite the appropriate pilot command and operating conditions.
Typical symptoms
Valid electrical checks but the unit remains in the wrong mode.
Checks that distinguish this cause
Use the OEM pipe-temperature, pressure and operating-state procedure to distinguish pilot action from main-valve routing. Do not hammer or force a valve as a diagnostic shortcut.
What can look similar
Incorrect command logic or inadequate operating pressure conditions may prevent an otherwise healthy valve from shifting.
Repair direction
Replace the confirmed failed assembly using approved refrigeration procedures.
Verify the repair
Verify actual refrigerant routing and capacity in each mode.
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.