Thermostat Digital Screen is Blank - Is it the AC Unit or the Furnace? How do I troubleshoot?

LWelsh

Premium Member
Joined
Nov 11, 2018
Messages
8
Location
Upland, CA
Model Number
80UHG Series
Brand
-major brand-
Age
More than 10 years
80UHG Series

The thermostat runs both the AC unit and the furnace. There is no reading on the digital screen. I used a voltmeter to determine that it is out because no voltage is going to the thermostat. I've researched a bit and found out it may be due to a blown fuse in one of the units. How do I know which one it is? The AC unit is a Tempstar model N4A348AKB300 (approx 5 years old) and the furnace is a Lennox 80UHG Series that's approx 15 years old.
 

rickgburton

Appliance Tech - Moderator
Staff member
Joined
Jul 11, 2006
Messages
33,423
Location
Murray, Utah 84107, United States
I used a voltmeter to determine that it is out because no voltage is going to the thermostat.
You'll need to look for the thermostat transformer. It's probably located on the control board for the furnace. Check for 120 VAC input and 24 VDC output. If the output voltage is correct at the transformer and not the thermostat there's a broken t-stat wire. If the output voltage is not correct you'll need to replace the board.
 
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