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AC Won't Cool - Diagnostic Tree

Troubleshooting Troubleshooting Updated Jun 19, 2026 · HVAC

Symptom

Customer reports the air conditioner is running but the house isn't cooling, OR the system isn't running at all and the house is hot. Most common service call from May through September. Correct diagnosis on the first visit prevents a callback and lets you size the repair quote accurately.

Likely Causes

In rough order of frequency for residential service:

  • Run capacitor failure - 30 - 40 % of "no cool" calls
  • Frozen evaporator coil from dirty filter or low refrigerant - 15 %
  • Tripped breaker / blown fuse at the disconnect - 10 %
  • Failed contactor - 8 %
  • Refrigerant leak (low charge) - 7 %
  • Failed fan motor (indoor blower or outdoor fan) - 7 %
  • Thermostat mis-set or low battery - 5 %
  • Failed compressor (the expensive one) - 4 %
  • Frozen line set or restricted metering device - 3 %
  • Other (drain switch trip, control board, etc.) - 11 %

Diagnostic Steps

  1. Verify thermostat setup. Mode set to COOL, setpoint below room temp, fan AUTO, batteries fresh. Skip nothing - surprisingly common cause.
  2. Confirm breaker + disconnect. Open the indoor air handler breaker AND the outdoor disconnect. Both should show closed; both fuses should test continuous.
  3. Listen at the outdoor unit. Hum without spinning = capacitor or seized motor. Clicking from the contactor without engaging = control voltage issue. Silence = no power getting there.
  4. Pull the outdoor service panel. Visual inspection:
  • Bulged or burst capacitor - replace
  • Burnt contactor contacts - replace
  • Burnt insulation on wiring - find the short
  1. Check filter at the indoor unit. A filter you can't see through restricts airflow → evaporator freezes → no cooling delivered. Replace + let thaw before further diagnosis.
  2. Inspect the evaporator coil and condensate pan. Ice on the coil = restricted airflow OR low refrigerant. Standing water in the pan = drain blockage; check for a tripped float switch.
  3. Gauge the system. With the outdoor running and airflow restored:
  • Low pressures + high superheat → undercharged (leak somewhere)
  • High pressures + low superheat → overcharged (rare in service)
  • Both low + indoor airflow restored → metering device restriction
  1. Measure indoor blower amperage. If above FLA, the motor or wheel is bad; if zero, the motor or capacitor is dead.

Resolution

Match the resolution to the diagnostic finding:

  • Capacitor bulged or out of spec → see "Replace an AC Run Capacitor"
  • Frozen coil + dirty filter → replace filter, thaw, recheck system after 1 hour
  • Frozen coil + clean filter + low gauge → leak search + repair + recharge
  • Failed fan motor → match HP + RPM + rotation; check whether the OEM is a dual-shaft or single
  • Low refrigerant without ice → leak-search before recharging; never just "top off" without finding the leak
  • Compressor seized → present replacement quote against the age + system warranty status

Never recharge a system without finding the leak first. A "top-off" charge that leaks back out in 30 days is poor practice, leaves the customer without cooling on the second hot day, and is an EPA Section 608 violation if you handle more than 50 lbs/year of refrigerant without leak-repair.

Prevention

  • Filter change reminders to every customer in March (before cooling season)
  • Annual maintenance visit catches a marginal capacitor before it dies in July
  • Surge protection on the disconnect prevents storm-induced contactor + capacitor failures

References

  • Manufacturer service literature for the specific equipment
  • EPA Section 608 refrigerant handling regulations
  • Manuall internal: Annual HVAC System Maintenance, Replace an AC Run Capacitor, Heat Pump Refrigerant Charge Reference

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