Hello All. I have a Honeywell TH9421C1004 VisionPRO IAQ thermostat installed in 2008 and the LCD display screen stopped working and Honeywell has discontinued the product. There are only three wires (red, white and blue) that connect to the thermostat (see picture). I am not the original owner of the house and looking for a replacement. I 've tried replacing the unit with simpler unit from a big box store with no success. What am I missing? I just need the thermostat to turn on the AC, furnace and fan. Any feedback would be greatly appreciated
That stat communicates with one more component before your furnace called an EIM module. You have to either get a newer model of stat that is still compatible with that module to keep the same configuration and simplicity of wiring to keep everything working the same, or you have to bypass the EIM module completely which will involve possibly running more wires up to the stat location and re-wiring the low voltage down at the furnace.