Thermostat Help - No Cool After Reset

Thermostat was heating and cooling for months but had multiple network connection issues. Finally reset the thermostat and suddenly the fan works but it does not kick on the outside unit.

Issue has persisted for weeks, living off upstairs unit cooling. Had tech check system, says it’s the thermostat and recommended changing. Outdoor unit can be turned on manually, power good at all connections, board working properly. Wires connected properly, checked all wires. When powering on the system, will briefly send 24 volts to Y1 and immediately drops to near zero. Voltage good at thermostat red to yellow and red to white at all times.

I am sure I selected the wrong settings when resetting the thermostat but have no idea what to put to get the system to kick on the outside unit. Maybe there is a different wiring configuration needed? I’m at a loss.

Gas furnace heat works, no problem there, only cooling. Attached are pictures (can only put 1 picture as new user…added pic of wiring of inside unit). Wires at thermostat are red (Rc), yellow (Y1), White (W1), green (C). Please help

Here is a picture of the original wiring, before installing the thermostat. Not sure if this helps any.

Because you have the C adapter, when you reset the unit, you need to tell it your “old thermostat” has the following wires: Rc, Y1, W1, G


I’m looking closely at the c adapter and it is looking like it is improperly wired.
Here’s the proper wiring diagram using your wire colors,
Please rewire the ac contactor so that it is connected to the furnace and not to the c adapter.

Just to confirm, you’re suggesting moving the wires as described in the edited image below? Yellow wire from C Connector to Y1 and Red wire to COM 24v?

That red wire should be paired with a white wire that goes to the Y terminal. they should be on the furnace side of the C adapter, not attached to the terminal strip on the C adapter.

Got it, I’ll try that. And I’m understanding that I should not move the yellow one at this time. Please let me know if I’m mistaken, just a little confusing since the diagram provided shows the yellow and red going straight to the furnace.

the pale yellow/ cream color i use on my diagrams is meant to indicate a white wire, as the background is white, using an actual white line there makes it pretty much invisible.

the wire that comes from the same sheath as the red wire is the other wire that runs to the air conditioner contactor that turns the outside unit on. that’s the wire that needs to be on the furnace’s Y terminal.

I understand. I will follow up with an update. Thank you for your patience and detailed instructions. I know it’s tough on text.

Unfortunately this did not work. Now the blower will not turn on when heater is engaged and outside AC unit still won’t start when cool is engaged.

did you change the initial setup of your thermostat so that it knows that it has the C adapter installed? AKA: reset it and tell it that your old thermostat has the following wires: Rc, W, Y, G

you might have damaged the c adapter by wiring it in the other way if it still doesn’t work after it knows that it has a C adapter that it needs to send signals to.

But you also might have enough wires in your run to not need the C adapter, if you have a blue wire at your thermostat.

While I thought the system was working the first few months, I later realized it was just pulling the cool air from the upstairs AC. Fan would run, and furnace would fire, but no AC.

The system needed to be re-wired outside. ABCD wired outside did not match/work with the furnace Y/W/R/G/Com as displayed in the attached images.

Paid to have another tech come out, looked up wiring diagram of furnace and compressor, matched wiring furnace to compressor, and it started working. Several capped wires on outdoor unit needed to be connected and only a couple of the ABCD wires were used.

Watching the entire process (which I did), this was not a DIY solution… well, worth the risk of frying a board or causing damage to the system.

For communicating systems, if the initial wiring does not work, either

  1. Wire it as it originally was and buy the very expensive thermostat. Cheapest new one I found was $450 on eBay.


  1. Get a professional out there to rewire it.

Yes, switching from or to a communicating thermostat setup is usually a pretty detailed task that requires reading the manual for all equipment involved, and a high level of understanding of what you are doing. Probably best to leave it to the people that do it for a living.