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New Nest, replacing an old thermostat. Image of old setup: enter image description here

Nest setup: enter image description here

The Nest says "cooling" with the blue screen for about 1 second, then switches to "delayed", then just shows black with "set" but doesn't cool. The blue screen never comes back.

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Pics of the furnace:

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Any idea why it wouldn't be cooling? I plugged the old thermostat back in and it's working fine.

  • Can you add photos of the wiring diagram for your furnace, as well as the control wiring at that end, to your question please? – ThreePhaseEel Jul 4 at 19:05
  • @ThreePhaseEel - unplugging the blue/C wire makes it work now. but i added pics of the wires i could find in the furnace – Dallas Jul 4 at 21:03
  • Is there not a wiring diagram posted at your furnace somewhere? What make/model is it, even? – ThreePhaseEel Jul 4 at 21:40
  • I don't see a diagram.. looks to be a "York variable speed diamond 95 ultra" – Dallas Jul 4 at 21:50
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I chatted with Nest support. They said to troubleshoot by disconnecting the blue/C wire.

It worked fine after doing this. They said the blue wire probably wasn't properly grounded.

When I asked why it worked with the old thermostat, they said "Traditional and non smart thermostats are not sensitive enough to detect the power and voltage issues. They will only activate the AC or heating - even if there are power issues. Nest thermostat will detect sensitive power and voltage issues and prevent the AC unit or heating unit from turning on to prevent damage to your HVAC system." Would be nice if the unit would give information though...

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