Direct answer: If your heat pump will not switch between heating and cooling, start with the thermostat mode and fan settings, then check for a dirty filter, a tripped breaker, or a system that never fully shut down between calls. If the indoor blower runs but the outdoor unit stays in the same mode no matter what the thermostat says, the problem is often in the thermostat wiring, low-voltage controls, or the reversing valve circuit.
Most likely: The most common homeowner-level causes are thermostat setup issues, a stuck fan setting that makes the air feel wrong, a dirty heat pump filter restricting airflow, or a simple power reset problem after a breaker trip or outage.
First separate a true mode-change failure from a comfort issue that only feels like the wrong mode. A heat pump in defrost, a blower set to ON, or weak airflow can mimic the same complaint. Reality check: many 'stuck in heat' or 'stuck in cool' calls turn out to be thermostat or airflow issues, not a failed outdoor unit. Common wrong move: flipping the thermostat back and forth every minute and never letting the system finish one clean cycle.
Don’t start with: Do not start by replacing major heat pump parts or opening electrical compartments. On this symptom, guess-buying parts wastes money fast.
The thermostat is set to HEAT, but the air feels cool or the outdoor unit behaves like it is still in cooling.
Start here: Confirm the thermostat is actually in HEAT, the fan is set to AUTO, and the set temperature is several degrees above room temperature.
The thermostat is set to COOL, but the supply air feels warm or the house keeps heating.
Start here: Set the thermostat to COOL, lower the setpoint well below room temperature, and wait a few minutes to see whether the outdoor unit and blower restart in a new cycle.
The display responds, but the equipment keeps doing the same thing as before.
Start here: Check for a blanked-out schedule, dead thermostat batteries if your model uses them, or a recent power interruption that left the system needing a full reset.
Some rooms feel drafty, weak, or uneven, so it is hard to tell whether the system actually changed modes.
Start here: Inspect the heat pump filter and make sure return grilles and supply registers are open and not packed with dust or blocked by furniture.
A thermostat set to the wrong mode, left in a schedule override, or running the fan continuously can make a normal system feel stuck in the wrong season.
Quick check: Set FAN to AUTO, choose HEAT or COOL manually, move the setpoint 3 to 5 degrees past room temperature, and wait through one full startup.
A dirty heat pump filter or blocked return can make heating feel cool and cooling feel weak, which gets mistaken for a failed mode change.
Quick check: Pull the filter and inspect it against a light. If it is matted with dust, replace it before doing anything else.
After a breaker trip, outage, or rapid thermostat changes, the indoor and outdoor sections can fail to restart cleanly or may sit through a delay that looks like a stuck mode.
Quick check: Check both the indoor air handler breaker and the outdoor disconnect or breaker, then do one controlled reset and give the system several minutes.
If the thermostat is calling correctly and the system still stays in one mode, the heat pump may not be shifting the refrigerant flow the way it should.
Quick check: Listen for the outdoor unit starting while the delivered air never changes character after a proper thermostat call. That points away from filter issues and toward controls or the reversing valve circuit.
You need to know whether the heat pump is truly ignoring a mode change or whether the thermostat never sent a clear call.
Next move: If the system starts and the air now matches the selected mode, the problem was likely thermostat setup, fan setting, or short-cycling from repeated changes. If the thermostat clearly changes modes but the equipment behavior does not, keep going. You have ruled out the easiest false alarm.
What to conclude: This separates a control-setting issue from a real equipment response problem.
Weak airflow is one of the fastest ways to misread what a heat pump is doing. A system can be in the right mode and still feel wrong at the vents.
Next move: If the air now feels clearly warmer in heat or cooler in cool, the heat pump was likely struggling against restricted airflow, not failing to switch modes. If airflow is decent and the air still acts like the old mode, move on to power and reset checks.
What to conclude: A dirty filter can blur the symptom enough that homeowners chase the wrong repair.
Heat pumps have indoor and outdoor sections. If one side lost power or the controls locked up after an outage, the system may not respond normally to a mode change.
Next move: If the system now changes modes normally, the issue was likely a control reset problem after a power interruption or rapid cycling. If the indoor blower runs but the outdoor unit still seems stuck in one behavior, the problem is no longer a simple reset issue.
At this point you are looking for physical clues that separate a thermostat signal problem from a heat pump that cannot actually reverse.
Next move: If the system clearly behaves differently and the air changes with the selected mode, the issue may have been timing, thermostat setup, or airflow and is now resolved. If the thermostat changes but the outdoor unit sounds and performs the same in both modes, the likely problem is in the thermostat signal path, defrost or control logic, or the reversing valve circuit.
This final check keeps you from buying the wrong thing. On this symptom, the thermostat is the only realistic homeowner replacement branch, and only when the evidence supports it.
A good result: If replacing a clearly failed thermostat restores normal switching, verify several full calls in both HEAT and COOL before closing up.
If not: If a known-good thermostat still does not change the mode, stop DIY and have the heat pump professionally diagnosed.
What to conclude: This narrows the safe homeowner repair path to the thermostat and pushes the higher-risk refrigerant and control side to a service call.
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Start with the thermostat. If it is really set to HEAT, the fan is on AUTO, and the setpoint is above room temperature, then check the filter and power to both indoor and outdoor units. If the thermostat changes but the outdoor unit still behaves the same, a control or reversing valve problem is more likely.
Yes. Restricted airflow can make heated air feel lukewarm and cooled air feel weak, so the system seems stuck when it is actually just moving too little air. That is why the filter check comes early.
Only if the thermostat itself is acting wrong. Good clues are an erratic display, lost settings, ignored manual commands, or inconsistent calls after a reset. If the thermostat appears normal and the outdoor unit stays in one mode, the thermostat is not the best first guess.
Not usually. Homeowners often assume that first, but thermostat setup, fan setting, airflow, or a simple reset issue are more common. A reversing valve or its control circuit moves higher on the list only after those checks are ruled out.
In most cases, no. That side of the diagnosis gets into live electrical testing and refrigerant-system work. It is a service call, not a safe guess-and-swap repair.