My A/C unit was just checked last week- the tech said it had enough freon. Monday we had a nasty storm- electricity flickered, but never went out. After the storm, we noticed the a/c went on, but it doesn't cool. Checked the thermostats and the circut breaker- both are working correctly. What can be wrong? The unit is 22yrs old. The tech did say we are running on borrowed time. He did mention replacing the capacitor and contactor.

  • When did "the tech" say that the time was borrowed? – Michael Karas Jun 11 '15 at 9:09
  • How have you verified that the thermostat is working? Is it turning the A/C unit on and off on the "cool" mode? – Michael Karas Jun 11 '15 at 9:11

It could be a variety of things, but there's no way to be sure without troubleshooting the system.


It could be that the thermostat isn't signalling the A/C unit to start.

Things to check

  • The indoor blower comes on
  • The outdoor unit turns on
  • There's voltage on the A/C signal wire from the thermostat

Control board

The control board in the air handler/furnace could be bad.

Things to check

  • The indoor blower comes on
  • The outdoor unit turns on
  • There's voltage on the A/C signal wire to the A/C unit


If the contactor is bad, the A/C unit will get the signal to come on, but the unit will not turn on.

Things to check

  • The outdoor unit turns on
  • There's voltage at the normally open contacts of the contactor (T1, T2, T3)


If the blower in the air handler/furnace is bad, cool air will not be blown throughout the home.

Things to check

  • The blower in the air handler/furnace comes on
  • Air flows from the supply ducts

Condenser fan

If the condenser fan isn't working, the system will have difficulty removing heat from the refrigerant.

Things to check

  • The fan on the outdoor unit (condenser fan) comes on


If the compressor in the condensing unit is bad, the system will not move refrigerant through the system.

Things to check

  • The refrigerant lines are different temperatures
  • The compressor turns on (may have to touch the unit to tell)

Restricted air

If air isn't moving through the system properly, it won't work well.

Things to check

  • Air is being moved by condenser fan
  • Condensing unit coils look clean and free of debris
  • Air handler/furnace filter(s) are clean
  • Air is flowing from the supply ducts
  • Evaporator coils are clean and free of dust and ice

Refrigerant level

If there's not the correct amount of refrigerant in the system, it will not function properly.

Things to check

  • Refrigerant lines are at the proper operating temperature (requires knowing the operating temperature)
  • Operating pressure of the refrigerant lines (requires a set of gauges)
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