Troubleshoot robertshaw thermostat 9420

Replacing a thermostat with a different model requires careful consideration of compatibility, wiring, and features. To determine if a Robertshaw 9560 can replace a 9420:

1. Verify compatibility with your HVAC system based on manuals.
2. Check wiring compatibility or consult an HVAC technician.
3. Compare features to meet your heating/cooling needs.
4. Ensure programming aligns with your requirements.
5. Confirm power requirements match your setup.
6. Assess the user-friendly interface.
7. Check thermostat size and mounting options.
8. Prefer professional installation if unsure.
9. Investigate warranty and support for the 9560.

Robertson 9420 is a multistage thermostat designed to control 24V heat pump systems,while the Emerson does that and more. So it seems it should be a possible replacement. However, the wiring may be more complex, depending on the layout and system.
Here is the info on both:

Did you check for voltage at the unit outside? Also check if the furnace/ac switch for the unit in the basement is in the on position?

Same thing happened to mine, it was replaced and the new one did the same thing about a year later. Needless to say it was no longer under warranty, which is no problem as I will not have another Robertshaw 9420 installed in my home. Very dangerous, especially if you are not home and it goes up to 99 or higher...

Hmm. I don't think it posted the first time around. So I'll try again.

There is a temperature sensing diode on the back side of this thermostat's circuit board. This diode may have failed or become disconnected.

I'd lean towards its having become disconnected, as I've been inside an example of this Robertshaw (originally Maple/Chase) 9600 thermostat and noticed that the soldering on this part was of poor quality. So if you have a soldering iron, and know how to use it, you could probably fix it.

I have a web page (just the facts, no ads, malware or fluff) that talks of some other common problems with this thermostat and how to fix them:

http://greyghost.mooo.com/maplechase9600/

thermostat gives the temp degree example 70 fl

my robertshaw 9420 wont let me set a temp and two snowflakes are flashing

have you checked to make sure you have a fresh battery in it

check with the company u brought it from and check if you have not put the correct conector to the right temals on the back of the switch.