If your heat pump is running but not cooling, the most likely causes are a reversing valve stuck in heat mode, a thermostat set wrong or miswired, or the same failures that hit any AC system: low refrigerant, a dirty coil, or airflow problems. A properly sized heat pump does cool a house at 115°F — that's not the source of the problem.
Why "it's just too hot outside" is usually wrong
This myth deserves debunking directly because it stops people from calling for help when they should. A heat pump's cooling capacity does derate somewhat as outdoor temperature climbs — the hotter the air outside, the harder the system works to reject heat, and the less capacity it has left over. But a heat pump sized correctly for your house through a Manual J load calculation is built to handle Arizona's summer peak. If your system is genuinely too small for the house, that's a sizing problem, not a "the desert is hot" problem, and it should have shown up as a struggle in June, not suddenly stopped working in August.
If your heat pump was cooling fine all summer and abruptly stopped or started blowing warm, treat it as a real failure and get it diagnosed. Don't wait it out assuming it'll fix itself once temperatures drop.
Reversing valve stuck in heat mode
The reversing valve is the part that lets a heat pump switch between cooling and heating using the same outdoor unit. If it's stuck or its solenoid has failed, the system can get locked in heating mode even with the thermostat set to cool, or it can run cooling only intermittently. This isn't a DIY fix — it sits inside the sealed refrigerant circuit, and a technician needs to verify it's actually the valve and not a control or wiring issue before replacing it. Reversing valve replacement typically runs $450 to $1,200; see our heat pump repair guide for the fuller cost breakdown across common failures.
Thermostat mode errors
Some smart thermostats and heat pump-specific thermostats have separate settings for heat pump mode versus conventional heat, along with O/B wire configuration that tells the system which terminal energizes the reversing valve. A wrong setting here can make the system think it's supposed to be heating when you've told it to cool, which looks identical to a mechanical reversing valve failure from the homeowner's side. Before assuming it's a hardware problem, check the thermostat's mode setting and, if it's been recently installed or reconfigured, verify the O/B wire setting matches your equipment.
When it's the same failure set as any AC
Past the two heat-pump-specific causes above, a heat pump that isn't cooling can fail for all the same reasons a straight AC does: a dirty or frozen evaporator coil, a clogged filter starving airflow, a failed capacitor keeping the compressor or fan from running, or low refrigerant from a slow leak. None of these are unique to heat pumps, and diagnosis follows the same process a technician would use on any AC-only system. If your heat pump is running constantly and the house still won't cool despite cold air at the vents, that points toward ductwork, insulation, or sizing rather than the unit itself — see our guide on why your AC isn't cooling the house for that failure set in more depth, since it applies to a heat pump's cooling side exactly the same as any straight AC.
For the fuller cost breakdown across common heat pump repairs, including reversing valve and capacitor pricing, see our heat pump repair guide. And if you're weighing repair against a full heat pump replacement instead, the $5,000 rule covered there applies here too: multiply the system's age by the repair cost, and if it tops $5,000, replacement usually wins.
What to check before you call anyone
A few checks take less than five minutes and cost nothing. Confirm the thermostat is actually set to Cool and not accidentally switched to Heat or Auto in a way that's confusing the system. Check the air filter — a filter clogged with monsoon dust starves airflow badly enough to mimic bigger failures. Walk outside and look at the outdoor unit: if it's iced over during summer, that alone points toward low refrigerant or restricted airflow rather than the reversing valve. None of these checks require opening the electrical panel or touching the refrigerant lines, so they're safe for anyone to do before calling a technician.
Common questions
Can a heat pump cool a house properly in 115-degree heat?
Yes, if it's sized correctly for the home. Cooling capacity derates somewhat as outdoor temperature rises, since the system works harder to reject heat into hotter air, but a heat pump matched to your house through a proper load calculation is built to handle Arizona's summer peak without failing.
Why does my heat pump blow warm air when it's set to cool?
The most common cause is a reversing valve stuck in heating position, though a misconfigured thermostat wiring setting can cause the identical symptom. A technician needs to check both before replacing the valve, since it's the more expensive of the two possible fixes.
Is a heat pump not cooling an emergency in summer?
Treat it that way in Arizona. A closed-up house with no cooling can climb into dangerous territory within hours during peak summer, especially for kids, older adults, and pets. Get a licensed technician out the same day if possible rather than troubleshooting for very long on your own.
Next step
Rule out the thermostat setting first — it takes two minutes and costs nothing. Past that, get a licensed technician out rather than guessing at parts. Look up any contractor's license at azroc.gov before they show up, and use the quote form below to compare a couple, including options in Yuma where summer peak temperatures push equipment the hardest.