Few things are more frustrating than an air conditioner that runs all afternoon while the thermostat creeps upward. In Tucson, where a July afternoon can sit at 105 degrees for hours, "running but not cooling" is the call we get more than any other. The good news is that most of these cases come down to a short list of causes, and a few of them you can rule out yourself in ten minutes.
Here they are, roughly in the order we find them.
1. A clogged air filter
Tucson dust is relentless, and monsoon season makes it worse. A filter that would last three months in a milder climate can load up in four to six weeks here. When it does, the system cannot move enough air across the indoor coil, the coil gets too cold, and cooling drops off. Left long enough, the coil ices over completely.
What to check: pull the filter and hold it up to a light. If you cannot see light through it, replace it. If ice has already formed, switch the system to fan-only for an hour or two so it can thaw before you run it again.
2. A failed capacitor
The capacitor gives the compressor and outdoor fan motor the jolt they need to start. Heat shortens its life, and Tucson roofs and side yards are as hot as it gets. A weak or failed capacitor is the single most common repair we make in summer.
What you will notice: the indoor fan runs and you feel air, but the air is warm. Outside, the unit may hum without the fan spinning, or the fan may spin but the compressor does not kick on. This one is a quick, inexpensive repair, but it is not a do-it-yourself job. Capacitors hold a charge even when the power is off.
3. A dirty outdoor coil
The condenser coil outside sheds the heat your system pulls out of the house. When it is coated in dust, cottonwood fluff or grass clippings, it cannot shed that heat efficiently. The system runs longer, cools less, and the compressor works far harder than it should.
What to check: look at the fins on the outdoor unit. If they are grey with dust or packed with debris, they need cleaning. A gentle rinse from the inside out with a garden hose helps, but bent fins and heavy buildup are better handled during a maintenance visit.
4. Low refrigerant from a leak
Refrigerant does not get used up. If a system is low, it is leaking. A low charge means the indoor coil cannot absorb enough heat, so the air coming out of the vents is only slightly cooler than the air going in. You may also see ice on the copper line at the outdoor unit.
Topping off a leaking system is a short-term fix at best. We find the leak, repair it, and then charge the system to the manufacturer's specification, because an overcharged system performs as badly as an undercharged one.
5. A frozen evaporator coil
Ice on the indoor coil is usually a symptom, not the disease. It comes from restricted airflow (a clogged filter, a collapsed duct, a failed blower) or from a low refrigerant charge. Either way, the ice blocks airflow completely and the house heats up while the unit runs.
If you find ice, turn the cooling off and let it thaw. Then find out why it froze, or it will freeze again within a day.
6. A clogged condensate drain
Your system pulls moisture out of the air, and that water has to go somewhere. When the drain line clogs with algae or dust, water backs up, and most modern systems have a float switch that shuts the unit down to protect your ceiling. You will hear the thermostat call for cooling and nothing happen, or the system will cycle on and off.
Clearing the line is part of every tune-up we do, and it is one of the reasons we recommend a visit before monsoon humidity arrives.
7. Thermostat and power problems
Before assuming the worst, check the simple things. Is the thermostat set to cool and the setpoint below the room temperature? Are the batteries fresh? Has the breaker for the outdoor unit tripped? Is the disconnect switch next to the outdoor unit in the on position? Half a dozen times a summer we arrive to find a breaker that tripped during a storm.
When to stop troubleshooting and call
If you have replaced the filter, confirmed the thermostat and breaker, and the outdoor unit is either silent, humming, or blowing air that is not hot, the problem is electrical or refrigerant-related and needs a technician. Running a system with a failed capacitor or a low charge can take out the compressor, which turns a modest repair into a replacement decision.
We diagnose the whole system, not just the symptom, and we quote the repair at a flat rate before we start. The diagnostic is free with any repair. Schedule a service call online or call (520) 261-3909. Same-day appointments are usually available across Tucson, Marana, Oro Valley and the surrounding area.