[Ehong]

My honeywell wifi thermostat went blank while my heater was running yesterday and now my unit just blows air (no heat) and I can't shut off the unit unless I unplug the York furnace. Here's what I've checked so far but still can't figure out what's wrong and the cause of my blank thermostat. Would appreciate if anyone can give their expertise and suggestions, thank you in advance.

1. Changed battery on thermostat (it's powered by c-wire but changed battery just in case).
2. Used multimeter to get a low voltage VAC reading from the wall where my thermostat is installed - no voltage detected.
3. Checked circuit breakers. Not tripped. Checked voltage at breakers, confirmed breakers are working fine.
4. Checked fuse inside furnace - fuse is fine, not blown.
5. Checked furnace transformer. 120v coming in fine and there are two low voltage leads going out. One had 24v reading (#1 in picture) and the other had 0v reading (#2 in picture). Took a reading between #1 and #2 at transformer and got a 25 vac reading. Did not do a resistance test.
6. Terminal strip testing on the board:
R-C gave 1.2 vac reading
R-W gave 1.2 vac reading
7. York furnace light blinking green at intervals of four.

Tempted to just replace the transformer but it seems it is working fine since I can get a 24 vac when testing the two low voltage leads together. Also concerned it might go out again if something is wrong with the system. Also tempted to just put in a new fuse (even though it looks fine) or resort to replacing the thermostat but it's only 4 years old and if I'm not getting power at the thermostat wall, highly doubt the thermostat is the issue.

Does it sound like a bad board?

Appreciate any/all help, thank you so much!

 

[Geochurchi]

Hi, 24 VAC from terminals on transformer, no voltage between R C on the board, fuse is a possibility, what does the Blinking LED indicate on the legend ?

 

[beenthere]

Remove fuse from board, and ohm it. Sounds like a blown fuse.

 

[Ehong]

Hi, 24 VAC from terminals on transformer, no voltage between R C on the board, fuse is a possibility, what does the Blinking LED indicate on the legend ?

thanks for the reply. I checked the fuse and it looks fine but I’ll try to swap it out. The light legend doesn’t mention green lights, it only talks about red lights.

 

[Ehong]

Remove fuse from board, and ohm it. Sounds like a blown fuse.

Could the fuse be blown even if it looks perfectly fine and is not burned or broken?

 

[beenthere]

Yes, i've seen the fuse look okay, but when OHMed out, it was open.

 

[Ehong]

Yes, i've seen the fuse look okay, but when OHMed out, it was open.

I just tested the fuse and it gave a reading of 0.3 ohms so its working fine.

 

[beenthere]

Remove board, and look for a burn spot on it.

 

[Ehong]

Turned out to be a stuck limit switch. Called out an HVAC company and the guy tapped the limit switch a few times and got it to work and sure enough everything works fine now. Should have checked this and kicking myself for wasting $90 on the diagnostic call.