AC running but not cooling? Here's the diagnosis order.
5 most common causes ranked by frequency, with the symptoms that point to each.
1. Dirty filter or condenser coil
Symptoms: AC runs constantly but doesn't reach setpoint. Bills higher than usual. Possibly ice on the evaporator coil or refrigerant lines (severe cases).
Why: airflow restriction = poor heat transfer. The compressor works harder, runs hotter, and may eventually fail.
DIY check: pull the filter. If it's grey or you can barely see through it, replace it. Then look at the outdoor condenser coil — it should look like clean fins. Bent, dirty, or matted with debris? That's your problem.
2. Low refrigerant from a leak
Symptoms: warm air at the supply registers. Maybe a hissing sound at the indoor or outdoor unit. Possibly icing on the evaporator coil.
Why: AC needs a specific refrigerant charge to remove heat. Slow leaks happen at fittings, schrader valves, or coil surfaces.
Diagnosis: the tech needs gauges. Pressures below normal = low charge = leak somewhere. Locating it usually requires an electronic leak detector or UV dye + UV light.
3. Failed capacitor
Symptoms: outdoor fan not running, or running slowly. Compressor humming but not starting. Frequent breaker trips.
Why: the dual-run capacitor stores electricity to give the compressor and fan their starting kick. When it weakens (gradual) or fails (sudden), things won't start.
DIY check: not really. Capacitors store dangerous voltage. Tech checks with a multimeter, replacement is a $35 part and a 20-minute job.
4. Failed contactor
Symptoms: outdoor unit dead even though indoor unit is on and calling for cooling. No clicking sound when AC tries to start.
Why: the contactor is an electrical relay. After enough on/off cycles (usually 10+ years of summer service), the contacts wear out or weld shut.
Diagnosis: tech checks for 24V at the contactor coil and 240V across the contacts. Pretty quick.
5. Failed compressor
Symptoms: outdoor unit hums loudly but doesn't start. Trips breaker repeatedly. May have started running rough months earlier.
Why: the compressor is the heart of the system. Most failures are electrical (motor windings) or mechanical (failed valves or bearings).
Diagnosis: tech checks compressor windings with multimeter. Failed compressor = expensive repair. On systems over 10 years old, time to consider replacement.