Heat Pump Not Cooling? What It Means When It Runs but the Air Is Warm

Your heat pump is set to cool, it’s running, and the air from the vents is warm or just room temperature. Or the outdoor unit sits silent while the indoor fan blows. Or it heated fine all winter and now won’t cool at all.

Those all feel like one problem. They aren’t. Several different faults produce the same warm air, and the fix for one does nothing for the others. Here’s how a heat pump switches into cooling, what you can safely check, and what needs a technician.

How a heat pump switches into cooling

A heat pump uses one refrigerant circuit for both seasons. A reversing valve sends the refrigerant one way for heating and the other way for cooling.

On Carrier and Goodman systems, cooling is the powered position. Carrier’s service manual for its 24 and 25 series splits says that on a call for cooling the thermostat connects R to O, and the R-O circuit energizes the reversing valve into the cooling position. Goodman’s GSZ service manual says the same for its heat pumps: the valve is energized through the thermostat’s O terminal and stays energized as long as the thermostat is set to cool.

That’s why the thermostat matters so much. Goodman’s manual warns that care must be taken when selecting a room thermostat and points to the installation instructions for approved ones. If a replacement thermostat isn’t set up for the heat pump it controls, the valve can end up in the wrong position. Our heat pump thermostat wiring guide covers the O and B terminals in more detail.

Same symptom, different causes

It heats but won’t cool. That points at the changeover. The thermostat setup, the O wire between the thermostat and the unit, the valve’s solenoid coil or the valve itself can each cause it. Goodman’s service manual notes that a reversing valve can stick in the heating position, the cooling position or partway between. A valve stuck partway sends some of the hot discharge gas straight back to the suction side, so the system runs and cools poorly.

The indoor fan runs but the outdoor unit doesn’t. Then nothing is moving heat out of the house, and you get room-temperature air. Carrier’s troubleshooting steps for an outdoor unit that won’t run go in order: the 24-volt call from the thermostat, the pressure switches, the contactor, then the wiring, capacitor and compressor. Each of those needs a meter.

Both units run but the air isn’t cold enough. Now it’s about heat transfer. Goodman’s homeowner section says a dirty indoor coil or restricted airflow can cause poor operation, and that dirt, leaves or debris on the outdoor coil restrict airflow too. It also explains that even a small hole in the coil tubing can eventually lose a large amount of refrigerant, and lost refrigerant causes poor operation. Poor airflow can also freeze the indoor coil. Our guide to a frozen evaporator coil explains how a technician separates airflow from low charge.

From the living room, all of these feel the same. Telling them apart takes testing.

What you can check first

These are the steps in the “Before Calling Your Servicer” list in Goodman’s heat pump installation and owner guide. They’re safe and quick.

• Check the thermostat. Confirm it’s set to cool and the set temperature is below the room temperature.
• Wait 15 minutes. Goodman notes that some devices in the outdoor unit or in programmable thermostats hold the compressor off for a while and then reset on their own. Some power companies also install devices that shut off air conditioners for several minutes on hot days.
• Check the breakers. Reset a tripped breaker once. Goodman says that if it opens again within thirty days, you should call a qualified servicer. Repeated resets risk severe equipment damage.
• Check the disconnect switch near the indoor furnace or blower. Make sure it’s in the on position.
• Check the filter. If it’s dirty, clean or replace it. Goodman says to check filters at least once a month and never run the system without one.
• Clear the vents. Make sure supply and return grilles aren’t blocked by rugs, curtains or furniture.
• Look at the outdoor unit. Make sure it isn’t covered on the sides or top, and remove anything that can be safely removed. If the coil itself is caked with dirt, Goodman says to call a qualified servicer to clean it.
• Listen. Unusual noise from the outdoor unit is its own reason to call.

That’s where owner checks stop. Don’t open the electrical panel on the outdoor unit, and don’t touch the refrigerant lines. Carrier’s own valve test instructions tell technicians not to touch the lines and to read their temperatures with remote probes.

When to book a diagnostic

Book if the air is still warm after the checks above and a 15-minute wait. Book right away if the breaker trips again, the outdoor unit hums but won’t start, you hear grinding or loud buzzing, or you see ice on the indoor coil or the copper lines. If ice is there, our frozen coil guide covers what to do while you wait.

A technician checks the thermostat signals at the unit, then the contactor, capacitor and pressure switches if the outdoor unit isn’t running. If it is running, they check the valve coil voltage, line temperatures across the valve, airflow, and refrigerant pressures. Refrigerant work is a job for a technician, not an owner.

Here’s how pricing works. The $99 diagnostic is credited toward an approved repair, and a written estimate comes after diagnosis and before any repair work. Typical minor HVAC repairs, such as a capacitor or contactor, run $250 to $650, parts and labor. A reversing valve or a refrigerant leak is a bigger job, and you’ll see that price in writing before you decide. More on that in our capacitor replacement cost guide.

Send the brand, the model if you have it, what the thermostat is set to, and whether the outdoor unit runs. If the thermostat was replaced recently, say so. That helps us narrow it down before we arrive.

Book a heat pump repair visit or contact us. Local pages: San Ramon, Danville, Pleasanton and Walnut Creek. For the opposite season, see heat pump blowing cold air in heat mode and switching from cooling to heating in the fall.

Key takeaways

• A heat pump cools only when its reversing valve is in the cooling position. On Carrier and Goodman systems the thermostat powers that valve through the O terminal for as long as it's set to cool.
• Warm air in cool mode has several causes that feel the same from inside the house: thermostat setup, an outdoor unit that isn't running, a stuck valve, poor airflow or lost refrigerant.
• Goodman's owner checklist is short: thermostat setting, a 15-minute wait, breakers, the indoor disconnect, the filter, blocked vents and anything covering the outdoor unit.
• Coil cleaning, electrical parts, the valve and refrigerant belong to a technician. The $99 diagnostic is credited toward an approved repair, and a written estimate comes before repair work.

Related questions

Need help with your system? Call (925) 999-4095 or book a visit. Repair diagnostics are $99, credited toward the repair. Installation assessments are free.

Written by Andrew Kuznetsov. Andrew is the founder and owner of Bay Area HVAC Service (ADRIUM Service Solutions). He holds a California Contractor License (CSLB #1136642), EPA Section 608 Universal certification, and is factory-trained on Daikin (Houston, 2025) and Mitsubishi. He writes from direct field experience, not marketing copy.

Further reading

• Choosing a controller for your Mitsubishi system , Handheld remote, MHK2, wired control or the Comfort app: what to check before adding a controller to a Mitsubishi indoor unit.
• Mitsubishi heat pump lineup: which family fits your home? , Compare FX, GX, JX, HX and WX wall units, SUZ indoor styles and P-Series, then decide between a single-zone system and a multi-zone layout.
• Mitsubishi heating and cooling for an ADU, studio or shed office , Plan a separate building around its rooms, access and utilities before choosing a mini-split.