trouble shoot air conditioning thermostat
Test the thermostat

There's a couple ways to do this, the "professional" way, or the "caveman" way.


Label and remove the wire connected to the Y terminal on the thermostat.
Set your multimeter to test continuity, or resistance.
Set the thermostat to cooling, and reduce the set temperature until the unit starts to call for cooling (may hear a click, and the unit should indicate that it's calling for cooling).
Touch one of the meter probes to the R terminal on the thermostat.
Touch the other probe to the Y terminal on the thermostat.


The meter should show continuity, and/or a low resistance. If not, the thermostat is bad, or not configured properly.

If the thermostat checked out, reinstall it and move to the next test.

Alternatively, a caveman would simply touch the "yellow" wire (the wire connected to the Y terminal) to the "red" wire (the wire connected to the R terminal).

This should turn on the A/C. If not, continue to the next test.

Test the wiring

WARNING: This test requires you to open up the furnace/air handler, and work on live equipment.  If you do not feel comfortable, contact a local licensed HVAC technician.
Next, you'll want to make sure the signal is getting to the furnace/air handler.


Carefully open the furnace/air handler, to expose the control wiring.
Set your meter to test AC volts.
Touch one probe to the R terminal.
Touch the other probe to the Y terminal.  
Have a partner turn the thermostat to cooling mode, and reduce the set temperature until the unit starts to call for cooling.


Initially; when the thermostat is not calling for cooling, you should read about 24 volts.  When the thermostat calls for cooling, that should drop to 0 (or very near 0, depending on how sensitive the meter is).

If this checks okay, move to the next test.

Test the contactor

WARNING: This test requires you to open up the condensing unit, and work on live equipment.  If you do not feel comfortable, contact a local licensed HVAC technician.


Open the condensing unit to expose the contactor.


Using a nonconductive implement (broom handle, etc.), push the contactor closed.



If the unit does not come on, it's likely time to contact a local licensed HVAC technician.

If it does come on, move to the next step.

Test the contactor control wiring

For these tests, you can pull the disconnect for the condensing unit.


Label and remove the control wires from the contactor.
Set the meter to measure AC volts.
Touch one probe to each of the wires you just disconnected.
Have a partner turn the thermostat to cooling mode, and reduce the set temperature until the unit starts to call for cooling.


Initially you should read 0 volts. When the thermostat calls for cool, that should jump to about 24 volts.

If everything to this point tested okay, it's time to contact a local licensed HVAC technician.