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