Most likely causes (in order of likelihood)

  1. Dirty condenser coils — the most common cause and the most overlooked. Coils on the roof clog with bugs, dust, and cottonwood.
  2. Failed run capacitor — fan runs but compressor doesn't engage, or engages weakly. $20 part.
  3. Frozen evaporator coil — restricted airflow causes ice buildup, which blocks cooling completely.
  4. Low refrigerant — usually from a slow leak. Requires a certified tech with EPA 608 cert.
  5. Failed compressor — the expensive one. Suspect after capacitor is ruled out.
  6. Thermostat reading wrong temperature or failing to call for cooling.

Diagnostic steps (in order, free/cheap before expensive)

  1. Check the thermostat. Make sure it's set to COOL (not FAN), and set the target temperature at least 5°F below current room temp.
  2. Get on the roof. Pull the AC shroud and inspect the condenser coils. If they look gray, dusty, or matted with cottonwood, clean them with a soft brush and coil cleaner spray. This alone fixes a large percentage of 'not cooling' calls.
  3. While on the roof, listen for the compressor. With AC on cool, you should hear a low hum separate from the fan. No hum and no cold air? Suspect the capacitor.
  4. Check the evaporator (interior). If you see ice on the coil, shut the AC off and run fan-only for 2 hours to thaw. Then check filters and return-air pathway for blockage.
  5. Test the capacitor with a multimeter capacitance setting, or replace it preemptively — they're cheap ($15-$30) and fail constantly. Match the microfarad (μF) rating exactly.
  6. If coils, capacitor, filters, and airflow all check out and it still won't cool, suspect refrigerant. This is your tech call.

DIY vs. call a tech

Coil cleaning, filter cleaning, capacitor replacement, and thermostat testing are DIY for anyone comfortable on a roof and using a multimeter. Refrigerant work is federally regulated — only a certified tech can legally service it.

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