Does Honeywell TH9421C1004 thermostate have backup battery

go online and look under model number Honeywell has instructions

Hello Bob Compher,

It looks like GETTING that battery is the biggest challenge...

After that it simply unplugs (like a portable phone battery).

Your manual is listed (below)

For assistance with this product please visit or call Honeywell Customer Care toll-free at
1-800-468-1502

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Honeywell VisionPRO IAQ Installation Manual Page 8

That model is hard wired to a controller and gets the power from that connection.

We'd love to help resolve this. Please give us a call at 1.855.733.5465 and our customer support team will be happy to troubleshoot with you.

On Honeywell thermostats you have to program the thermostat on the setup mode to have the auto to be on the display. The setup instructions will walk up through it. If you do not have them, you can down load them to you computer.

Make sure there is 24 Volts between the R & Common wires connected to the thermostat subbase.

check your float switch. On your air hander you have 3/4 white pvc coming out of bottom of unit, where the water drains out. If you have a float and your drain line is plugged it will make your tstat go blank. vacuum the drain line where it comes out of the ground outside.

That thermostat must be used with a THM5421C1008 Equipment Interface Module. The thermostat communicates with the module and the module is connected to the HVAC components.