Thermostat AC Compressor will not turn on

Just installed thermostat. Followed color codes exactly on thermostat side. I have all 5 wires hooked up on thermostat Y, W, G, C, Rc. I checked the furnace and the blue wire C was not connected, it was just wrapped up. I added the blue wire to C at the furnace( which also has the white wire going to the AC out side) . The other wire going outside to the AC compressor is also connected to the Y terminal. Heat works fine it appears. This is a Lennox system.

If you have a multi-meter, with the thermostat calling for A/C, is there around 24VAC between the Y and C terminals on the air handler? Be careful to not short anything out and make sure your meter is in VAC mode not resistance mode.

I do not have a multi meter. Does it matter that the previous thermostat was working fine? The only change I have made is connected the C wire to the furnace. Do I need to possibly use the C adapter instead of using the C wire directly to the thermostat? Not sure I completely understand but at least adding the C wire to the furnace initially gave the wyze thermostat power to turn on. It would not power on before that. Below is a picture before I added the additional C wire.

Here is my original thermostat setup. The blue wire was not connected at the furnace (as noted in the previous picture) but wired in at the thermostat.

maybe a bad connection on Y at the Wyze tstat?

I will check that and let you know.

Connection was solid for Y at the tstat. Pulled wire and attached. Did not resolve.

I’m trying to determine if the wyze is sending the signal out on the Y terminal and the A/C is just not responding, or if the wyze has a defective output on Y.

To test the Y output from the thermostat without a multi-meter, with the power off, disconnect the G (green) wire from the air handler (just leave it disconnected, but not touching anything for this test) and attach the Y (yellow) wire that comes from the thermostat to the G terminal on the air handler. Then, power up, and test cooling on your wyze. Does the blower fan start running? You may need to close the blower door in order for the system to power up, or temporarily bypass the safety switch by holding it down with with a piece of tape.

If the blower fan does not run, then you should turn the power off, put the Y and G wires back where they were originally on the air handler(yellow to Y, green to G), and go to the thermostat side and pull the Y and G wires out and reconnect the G wire to the Y terminal, leaving the Y wire disconnected and not touching anything else. Then power it back up and run the cooling test again. Does the blower fan turn on?

If the blower fan turns on in the first test, then your A/C compressor or the wires going from the air handler to the A/C has an issue - or your transformer may not have enough power to pull in the contacter on your A/C and power the wyze at the same time.

If the blower fan does not run in the first test, but does run in the second test, then your Y wire from the thermostat to the air handler is defective, sometimes they break inside the insulation, check near the thermostat where you may have had to bend the wire into a different position from the old thermostat.

If the blower fan does not run in either test, then your wyze thermostat has a defective Y1 relay and you will need to contact support and get them to ship you a new one.

Remember to power down the system and put your wires back where they were after the tests. If you do not power down your system when you are adjusting the wires, you run a high risk of blowing the fuse on your air handler.

FYI, the blower fan has always worked… just the outside compressor does not turn on. And, if it simplifies things, I have an identical system and another wyze thermostat that I have not started working on yet

However, unless you say different from my comment I will follow your steps.

That’s fine, we’re just using the blower to test the Y output. Follow the steps, the outcome will show where the problem lies.


I will be back home later this afternoon and will test then. Thank you for your involvement.

Ok. I pulled the G wire coming from the thermostat, and going into the furnace, out and left it unplugged. I took the Y wire coming from the thermostat and plugged it into the G terminal att the furnace, as instructed. When I powered on and turned on Cool the blower did come on.

Also, as an FYI, keep in mind that I have a completely separate identical system that I have not started to work on yet to do the same thing with a second thermostat that I have. In going forward, if the assumption that there may be a problem I can try a setup on the second system, assuming when we run out of other options to try.

Then there is either something wrong with your AC compressor, or your transformer isn’t strong enough to pull in your AC contractor while it is powering the wyze thermostat.

Could it possibly be related to the outside temperatures being under 50 degrees? Wondering if Lennox has a protection that keeps the compressor from cutting on when it’s cold outside. I have two separate systems one I was working on for the Wyze and one I was not. Both systems are not turning on the compressor with either the Wyze or the old thermostat. The systems are 3 years old.

Oh, the old thermostat is not turning on the compressor either?
Probably has a low ambient temp lockout if it’s happening to 2 different units. Minimum safe operating temperature with certain compressor oils is around 60F.

Thank you speedie for all your help. Your involvement, advise, and opinion has provided me confidence in the product and in my case may just be environmental conditions that are limiting our ability to fully test. I am hopeful when it warms up a bit that the compressor will come on. Otherwise, the thermostat is working fine in operating the heat.

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Update. It has warmed up and the compressors do not cut on. I do not believe it’s an outside temperature issue. One observation I have, when I turn on the ac I hear a click and then a slight hum on the outdoor unit. When I turn off the ac I hear a click and the hum goes away. Any thoughts?

You might have a dead capacitor or contactor on your outdoor unit. You will need to call a HVAC professional to diagnose and replace the appropriate part, it’s not something I recommend you do as it is dealing with the high voltage side of things.

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