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I recently installed a nest thermostat for our AC system. Unfortunately I was short sighted and just assumed all the wires were the color I needed. Since installing the thermostat our system won’t cool. The fan runs and the unit outside runs but no cold air. The pictures show the wiring on out unit in the attic and how it’s wired into the nest. I’m confused why one wire says heat even thought this is AC only. Did I mess up the wiring or is it something else (low on Freon/bad compressor)? Thanks for your help!

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  • Can you post a wiring diagram for your air handler? – ThreePhaseEel Jun 2 at 22:16
  • It’s a Lennox cbs18-26-1p, I haven’t been able to find the exact wiring diagram but will look again – Tim Rozendaal Jun 2 at 22:24
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    Assuming the system ran just recently, prior to upgrading to the Nest? Is this an upgrade and the system hasn't been used for a few months? Could you put the old thermostat back on the way it was and test the system to see if it cools. It sounds like the system is running correctly, and may be a coolant issue. If you have someone turn the system on, while you are outside, listen for 1: Compressor fan (should be able to see this too), and 2: The compressor to kick in (should be noisier than just the fan, almost like an air sound as a compressor would sound if you are filling a tire or similar). – Jeff Cates Jun 2 at 22:30
  • I did wire the old one back up and didn’t notice it cooling either. I ran the ac for a while and the fan outside will run immediately but the condenser does not appear to kick in. My main question I guess is if I am wiring in the white wire wrong causing the condenser not to run since white appears to be for bear according to the nest – Tim Rozendaal Jun 2 at 22:43

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