Hello all. I replaced our old turn dial thermostat with a programmable digital thermostat (Honeywell thx6400b). I turned off the breakers for the house prior to opening up the old thermostat, where I found a green, yellow, red and white wire attached. I removed the old t-stat and hooked up the wires to their corresponding spots on the new one (which includes an Rc and Rh input with a wire jumper between the two... ). However, when I turned on the power, only the fan and heat settings work - the air being blown out is not being cooled. It worked perfectly before my tstat swap.

I checked the compressor and it is no longer running. None of the circuit breakers in the breaker box are tripped, and the fuse on the actual furnace/control board unit in our basement is fine. The problem persisted even when I reinstalled the old tstat, so I assume I must have done something to the compressor itself. When I short the red and yellow wires together, the compressor will make a clicking noise, but does not start up even after waiting for hours (I understand that most units have a delay)... instead, the blower simply pushes luke warm air out the registers. I have opened up the compressor unit's back panel, but do not see any fuses or breakers to check. There is a power switch next to the compressor (mounted on the outside wall), but no outside breaker next to it At this point, I have simply disconnected the tstat wires and am letting the unit sit idle, hoping that it might simply have an overload shutdown that was tripped and needs to sit for an hour or two before resetting. In the meanwhile, any ideas what might be wrong or what I should try next?

Not sure if this changes the diagnosis, but the compressor is a Lennox 10ACB48-11P. I do not havea meter reader to determine whether the various connections are at the correct voltage.

Thanks in advance for your thoughts.

Sounds like you need to re-look at your wiring
Like the wire that goes to the ac
With the jumper that sends power to ac if wires are in right spot

To clarify "what does turning off the house breakers" mean?

Turned off all of them? The main?
Just the air handler?
The condensing unit and the air handler?

The R to Y click caused a click to be heard in the outside unit. Yes? No?

If your comfortable doing this, it's worth a try. Using the outside disconnect, disconnect the outside unit. Usually it's a pull out with an on and off position. You basically pull, rotate and insert it back again. The on position usually has a higher insertion force.

So, pull the disconnect.

Take off the cover of the condenser and trace the two small tstat wires and the power wires. They should terminate at a relay (contactor) and it probably has exposed contacts.

You can leave the disconnect off for the next test. When there is a call for cool the contacts should touch and it should click.

I bet it does. Does it?

If yes, (assuming nothing changed) then the likely cause is no 240 VAC. You can turn the tstat to off and put the disconnect back in, in the ON position and test, but I'll bet you get the same result. Click, but unit won't turn on.

Now if, you didn't turn off power to the outside unit (A high amperage connected breaker) when you replaced the tstat, then you may have a broken breaker. Try to reset it and try again.

When you don't wait for 2 minutes when engaging the compressor, large surges can develop and take out the breaker.

Your going to need a meter or a test light to troubleshoot further plus the skills not to get shocked.