What happens
A documented solenoid circuit fails to actuate the compressor's stage mechanism.
Typical symptoms
High-stage demand arrives but the expected capacity change does not occur.
Checks that distinguish this cause
Use the exact compressor procedure for its solenoid connector, drive signal and isolated tests. Do not assume a generic 24 VAC coil or apply external power to an unknown assembly.
What can look similar
A missing controller output or refrigerant condition can produce no capacity change with a healthy solenoid.
Repair direction
Replace only the approved serviceable failed component; some faults require a larger assembly replacement.
Verify the repair
Verify both stages and their specified airflow and refrigeration performance.
Equipment-specific limits
A valid Y2 call is only one part of staging. Solenoid voltage, timing and operating-stage logic depend on the exact compressor and controller.
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.
- Copeland: Two-stage scroll compressorsLow-voltage solenoid modulation on applicable two-stage scroll models.
- Resideo: RTH6360 thermostat installation and setupExamples of heat-pump O/B and staging setup. Menu numbers vary by thermostat.
Editorial review: 2026-09-29. Verify the current manual for the installed model.