Before I begin, I would like to request "call a technician" suggestions to stay off. I do know a nice guy who can look it up for me, for a variety or reasons, one of which being my general history of fixing stuff, I would like to take a first stab at solving this problem. Thanks

Background: Both units are middle-of-the-line Lennox, about 2 years old. The thermostat is Nest 1st generation, which also controls the completely separate hydronic heating system used with radiators. The system does only AC (is not a heat pump, in other words).

Problem: I turn the AC cooling on and only the indoor unit (air handler) fan comes on. The outdoor unit (compressor) stays off. I checked power supply at the main panel (both units have a dedicated circuit, 15A for the handler, 30A for the condenser) and using a pen-like current detector.

Wiring Pictures

1. Inside Air Handler

Inside Air Handler

Note that the upper thermostat cable is what goes to the compressor, lower goes to the thermostat. The two white wires are from a small water detector attached to the pan in which the return box sits to switch the system off in case water drips in the pan.

2. Thermostat


Note that the RH red wire and the W1 white wire come from the above mentioned radiator heating system and are completely unrelated to this. RC red goes to the R in the air handler.

3. Compressor

Compressor Compressor Info

So the C connector from the thermostat is connected to the air handler via the blue wire, which goes to C in the air handler and to which the blue from the compressor cable connects it to the black wire in the compressor. My suspicion is that somewhere along is the problem unless the compressor is broken, for example a problem with its capacitor or something else broken.

Question: How can I diagnose where the problem is using a multimeter? Is it possible to narrow the problem down to the capacitor OR the thermostat using my analog multimeter, i.e. to rule out the wiring, which seems like it is fine? I can turn the system on and I would like to use the multimeter to verify that the thermostat is properly turning the cooling on via the blue wire to the compressor. If so is the case, then the wiring is not the problem.

  • 1
    You mean 3. Compressor? Because that looks like the outside uint.
    – JPhi1618
    Apr 22, 2016 at 18:15
  • i've always called it a condenser. may be wrong. fixed
    – amphibient
    Apr 22, 2016 at 18:16
  • Yea, I was thinking evaporator. The condenser coil surrounds the compressor motor in the outside unit.
    – JPhi1618
    Apr 22, 2016 at 18:18
  • Has this ever worked? Did it stop working when you installed a new thermostat?
    – Tester101
    Apr 22, 2016 at 21:13
  • @JPhi1618 You mean 3. Condensing Unit.. The compressor is a device inside the condensing unit.
    – Tester101
    Apr 22, 2016 at 21:50

3 Answers 3


The first thing to do is check the two wires on the outside unit when the thermostat is calling for cooling. In your picture it looks like the Blue and Yellow thermostat wires are being used. You should have 24v AC across those two wires, which would normally close the 240v relay and start up the compressor and cooling fan.

If you're not getting 24v on the two thermostat wires, there's probably a bad connection somewhere. Since the air handler is working and the thermostat has power, the 24v transformer is working.

If you are getting 24v, then you can move on to diagnosing just the compressor. There are limit switches that can prevent the compressor from turning on if the coolant level is off. There can also be temperature sensors that prevent the compressor from running, so these would need to be checked. The relay should also snap shut when the 24v signal is present. A poor connection might read 24v, but might not have the amperage to actually close the relay. The relay can be corroded or filled with dead ant bodies.

I hope this helps. Clarify anything you can in the comments and I can attempt to edit as needed. I've done a lot to diagnose and solve problems with a WiFi thermostat and heat pump that were not wired correctly.

  • OK -- next question -- when i take one of the multimeter probes to the thermostat wire in the compressor, where should i put the other one ? Can i put it on the ground wire that goes to the grounding terminal ?
    – amphibient
    Apr 22, 2016 at 18:32
  • You have to measure both of the 24v wires (one to the red probe, one to the black). They won't be relative to any equipment ground. Disconnect the 240v at the cutoff switch and/or breaker and you won't have anything to worry about. The 24v won't shock you so you can safely take off the wire nuts and probe the thermostat wires.
    – JPhi1618
    Apr 22, 2016 at 18:38
  • I measured and there was nothing. But there was also nothing in the blue C connection inside the air handler (cooling is on, Nest thermostat is blue). Is there any way I can hack or jump the outside unit (whatever you call it), to make sure it works, which would mean the thermostat is the problem ? I have a small low voltage magnetic jumper I used to connect the R and G to turn the fan on.
    – amphibient
    Apr 22, 2016 at 18:54
  • On the thermostat connecting plate, jumping the red wire (Rc) to the yellow wire should turn on the outside unit. The green wire to the red should turn on the fan of the air handler. Also at the thermostat, there should be 24v between the Rc and C (blue) wires.
    – JPhi1618
    Apr 22, 2016 at 18:57
  • i can't test the wires inside the thermostat while the thing is running -- when i take the Nest from the plate, it turns everything off. i can do it inside the air handler
    – amphibient
    Apr 22, 2016 at 19:00

The compressor is in the condenser unit where the heat is removed in most cases. The evaporator is inside the air handler where the cold air is made for the house. If the cap is blown it will still try to start and normally blow the fuse. The cap can be checked out of circuit by using ohms and touching the leads you should see the value climb. Then reverse the leads and you should observe the same thing. It looks like you may have a dual cap look on the label to make sure then there would be 2 combinations to see the charge on the ohm meter. There is usually a contactor or relay that is controlled by the main unit or thermostat. If this contactor is not pulling in the power to the compressor and fan are not turned on to the compressor and fan motor.


I had the same problem many years ago. The air handler would come on but not the outside compressor or blower motor. Every breaker and connection checked out fine.

I watched the outside main contactor for the motors and noticed that the contacts were so burned away they could no longer work. I founds its part number and ordered it and replaced it.

The AC has been working fine ever since then, but I now know that Florida's humidity and rain limit the life of the contactor to about ten years.

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.