Long runtime on a very hot Tucson afternoon is normal, but an AC that never cycles off on a mild day, or that runs while the thermostat is off, has a fault. Climate Pros Tucson matches you with licensed local pros for the fault. First work out which case you have.

Is it normal for an AC to run all afternoon in Tucson?

Running all afternoon is often normal. Never cycling off is not. With about 68 days a year at or above 100F at the airport (1991-2020 normals), long runtime is expected in summer, and our guide to an AC that can’t keep up covers that case. The test for this page is simpler: does the system stop once the house reaches the setpoint, or when you switch the thermostat to Off? If it doesn’t, something is holding it on.

What makes an AC run when it should stop?

  • Stuck contactor. The contactor is the outdoor switch that starts the compressor. If it sticks closed, the outdoor unit keeps running even after the thermostat is satisfied.
  • Thermostat fault. A misplaced sensor, a low battery, a bad relay or a setting on Fan On can keep things running.
  • Frozen evaporator coil. Ice blocks airflow, so the house can’t cool and the system keeps going. Read AC freezing up and check the filter.
  • Low refrigerant or a leak. The system runs but can’t move enough heat. See AC hissing noise causes.
  • Undersized or struggling equipment. If it never reaches setpoint on a normal summer day, ask a pro to check sizing, airflow and ducts.

What can you safely check?

Set the thermostat to Off. If the indoor unit stops but the outdoor unit keeps running, switch off the AC breaker at the main panel and leave it off. That points to a stuck contactor, and a running compressor with no call for cooling can damage itself. Don’t open the outdoor disconnect box or touch wires. If the whole system stops when the thermostat goes off, look at the thermostat settings, the fan switch and the batteries, and see Honeywell thermostat troubleshooting.

When is it urgent?

Turn the system off at the breaker if you see ice that doesn’t clear, smell something burning or hear loud grinding. The CDC advises older adults to get into air-conditioned space in hot weather, so plan where to stay cool if the system must stay off. Read AC not turning on in Tucson for the opposite problem.

What will a pro do?

The pro you’re matched with checks the contactor, the thermostat wiring, the coil and the refrigerant side. Ask for the diagnostic fee and whether it applies to the repair, since each company sets its own policy. Check the license at roc.az.gov. See AC repair and the Tucson HVAC hub.

When it still won’t shut off

If the unit won’t cycle off, or the outdoor unit runs with the thermostat set to Off, a licensed pro needs to find what’s holding it on. Call (520) 353-2520 and we’ll match you with a licensed local pro.