Most likely causes (in order of likelihood)

  1. Failed run capacitor — the compressor needs a brief electrical kick to start. Without a working cap, the fan runs but the compressor never engages.
  2. Thermostat issue — set to FAN instead of COOL, or thermostat has failed.
  3. Compressor protection lockout — if the unit just lost power, it may be in a 3-minute restart delay.
  4. Failed compressor — terminal. Cheaper to replace the whole AC unit.
  5. Refrigerant loss from a leak — slow loss usually shows as poor cooling first, then no cooling.
  6. Low voltage at the pedestal — under 105V on a 30A connection can prevent compressor start.

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

  1. Verify the thermostat is set to COOL and set 5+ degrees below room temperature. Check batteries if it's a battery-powered stat.
  2. Wait 5 minutes after any power interruption before judging. Compressors have a built-in restart delay.
  3. Check pedestal voltage with a surge protector display or a multimeter. Under 105V is too low to safely run AC. Move pedestals or stop running.
  4. Test the capacitor. Pull the AC shroud, locate the run capacitor (usually a cylinder with 2-3 terminals), and test with a multimeter on capacitance setting. Any reading more than 10% off spec means replace it.
  5. Replace the capacitor preemptively if it's more than 5 years old or has visible bulging. Match microfarad (μF) rating exactly. Cost: $15-$30.
  6. If the capacitor is good and the thermostat is calling for cool, the next step is compressor testing — at this point, get a tech.

DIY vs. call a tech

Thermostat checks, voltage testing, capacitor replacement, and shroud removal are DIY. Compressor diagnosis and refrigerant work require a tech.

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