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I replaced an old Honeywell thermostat about a month ago with a Nest thermostat. Friday Nest started displaying an E74 error.

I have two furnaces and two outdoor units, one for upstairs and for downstairs. Upstairs works fine, downstairs does not.

Both furnaces show “normal operation” LED codes.

There are W1, C, Y1, Rh and G wires.

I have done the following to check what was going on with no different result:

  • Reinstalled old thermostat (no power if I remove batteries)
  • Connected Rh to Rc
  • Removed the C wire
  • Removed the Y1 wire
  • Made a connection with Rh to G to check if fan would come on
  • Replaced fuse on furnace (both furnaces, they weren’t blown but thought I’d try anyway)
  • Checked all breaker boxes, turned on and off repeatedly
  • Checked incoming power to AC unit outside, read 249V.
  • Recut and rewired Nest completely.

I have no idea what other steps to take, I am in Texas - it is currently relatively hot outside and I cannot find a professional until June 1st

  • Have you checked the low voltage transformer in the furnace? Depending upon which furnace...most are 24volt. They do fail. – George Anderson May 24 at 15:50
  • I haven’t checked the low voltage transformer. I’m going to research how to do that and I’ll update in a little while. – Somewhat Insane May 24 at 16:19
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    You really need a meter, if you don't have one. They aren't that expensive ($30 and up). I like ones with an "amp clamp" which enables you to measure current (amps). Don't bother with those proximity voltage testers, they can be easily misunderstood for those without much electrical experience. – George Anderson May 24 at 16:37
  • Do you have a voltmeter? If so, set it to AC volts and see if you can measure 24V between R and C – ThreePhaseEel May 24 at 16:37
  • Hey 3ph. I thought of that, but in the OP he said even when he reinstalled the old stat he had no power if he removed the batteries. So I thought, let's start at the source and see if he has power off the transformer. Still your suggestion is a good diagnostic step. – George Anderson May 24 at 16:42

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