Symptoms
- System stays in one mode (heating or cooling) regardless of the thermostat setting
- Switching modes on the thermostat doesn't change what comes out of the vents
- May have started suddenly or developed gradually
Likely causes
- CommonStuck reversing valveThis valve can stick mechanically or electrically, especially on older or infrequently used systems.
- CommonFailed reversing valve solenoidThe small electrical coil that moves the valve can fail even if the valve itself is fine.
- PossibleThermostat wiring or setting issueIncorrect wiring, especially after a DIY thermostat swap, can prevent the mode signal from reaching the valve.
- Less commonLow refrigerant affecting valve operationIn some cases, inadequate refrigerant pressure can affect how reliably the valve switches.
What to do
- Confirm the thermostat mode selection is actually changing (check the display reflects Heat vs Cool as expected).
- Cycle the system off, wait a minute, then try switching modes again in case it's a one-time glitch.
- If you or someone else recently replaced the thermostat, double-check the wiring against the base plate diagram.
- Note whether the system has been stuck in one particular mode for a while (like all summer) since valves can seize from lack of use.
- Don't attempt to access or manipulate the reversing valve yourself; it's a refrigerant-carrying component.
- Call an HVAC technician to test and, if needed, replace the reversing valve or its solenoid.
Tools and parts
Heat pump reversing valve solenoidSearch: “heat pump reversing valve solenoid replacement”
When to call a pro
Call an HVAC technician to test and replace the reversing valve or solenoid if mode switching doesn't respond after basic thermostat checks.
Safety
Use caution
The reversing valve is a refrigerant-carrying component; never attempt to open or service it yourself.