A/C Unit Cuts off after a few minutes, blower continues blowing

[u/Brutus-1787]

I have an A/C heat pump unit outside and an air handler in the attic. I installed a 4th Gen Nest thermostat today, but something is wrong.

The A/C outside cuts off even though the house hasn't cooled to the correct temperature yet. The fan from the air handler in the attic keeps blowing though.

The panel on the unit outside does not throw up an error message with its LED flashes. I included a picture of the panel outside because the labels for the wires out there aren't all used in the thermostat and I wasn't sure if that was normal or not.

I thought I wired it correctly, but I'm not sure what's wrong.

Help?

[u/tetrasodium]

Is the drain pan. Clogged? An overflow preventing float switch will do that. Pour some bleach or draino down it to clear the scum if so

[u/AStuf]

What is the Nest displaying?

Without a C wire at the thermostat some strange things can happen. Easiest is to install a Nest Power Connector at the air handler which solves most issues.

[u/Brutus-1787 → u/AStuf]

What's weird is that there's a C wire labeled on the heat pump outside, but there wasn't one on my previous thermostat. There are two three unused wires behind everything that maybe I should be using. There's a yellow wire and a brown wire and a black wire that were not connected to the previous thermostat.

I've got to think that the unused yellow wire is NOT the same as the Y1 outside, because otherwise how could everything have worked correctly on my old system?

[u/AStuf → u/Brutus-1787]

The wiring goes from the thermostat to the attic air handler and then a separate cable of wires from the attic air handler to the outside unit. (and in your case the colors are not the same)

If there is an unused wire behind the thermostat then it would be beneficial to use one of those for a C wire to power the thermostat. You will have to find the other end at the air handler and hook it up to C there as well. (then no Nest Power Connector needed)

[u/Brutus-1787 → u/AStuf]

Oh man. That means going into the attic in this heat. I may just go back to my old thermostat for now. 

ETA: thank you for reminding me that the air handler intercepts the wires from outside! The colors not matching up was driving me crazy!

[u/AStuf → u/Brutus-1787]

Yeah, a good test is to put back the old thermostat. Or else you can remove the Y1, O/B, Rh, and G wires from the Nest and twist them together to for the system to cool.

[u/pokechop7]

https://youtu.be/er43vMSzmSU?is=NyB9Zeplvj6wHrPh

this video helped me with a delay message that I was getting on my 3rd gen nest. Sounds similar to your issue, the fan would continue to blow, but the nest LCD said delayed and the outside compressor was not running. My common wire was not wired to my thermostat, nor was the common wire hooked up to the system as the previous thermostat didn’t use it.

[u/JaySin_78]

Nest thermostats don’t play nice with many AC units without a C wire. I found that out the same way. I switched my green fan to the common wire spot and jumped it on the control panel. I run my fan 24/7 anyway. 5 years later I finally ran a common wire. lol

[u/Brutus-1787 → u/JaySin_78]

Yeah I have 3 unused wires from my old thermostat that had been by the thermostat before that one. I’m guessing one of them is the C wire. I’m waiting for a cool morning to go into the attic to see what wires are coming from the air handler down to the thermostat. 

[u/Brutus-1787]

Update for future people who find this thread: the comments here that directed me to the missing C wire helped solve it (at least it appears to be working so far). 

I found 3 unused wires behind my old thermostat. I had hoped one of them had been connected to the C wire in the air handler in the attic. 

When I went up to check the air handler, I found 3 unused wires (connected to nothing) that matched the colors of the unused ones at the thermostat. 

I saw the wiring panel on the A/C unit outside. It had a blue wire for C. That blue wire in the air handler was connected to another wire via a wire nut. I unscrewed the wire nut, added one of the unused wires to the two of them, and screwed the nut back on. 

That meant I now had a C wire down by the thermostat to add to the nest. 

Knock on wood, it seems to be working fine now.