I would first check to see if the thermostat has been programmed correctly. Here are some things to check:

- are you in heating mode?
- are you in a set back period?
- is the time set correctly?
- was the hold temperature mode in use?

If you are convinced the set up is correct then inspect the back plate carefully. If seating and unseating it makes it work then you may have a contact connection problem. If everything looks good visually then the thermostat itself may be defective and needs to be replaced.

If this is dual fuel package system-HP with gas furnace backup, then you should have an outdoor temp sensor with this thermostat.

I would recommend replacing the thermostat and if in high humidity location in the summer, select the 8321 HW model.

IMO

Thanks for the quick replies.

We are in heat mode, clock was 20 minutes slow but not enough to affect anything, wake mode, not on hold. The contacts look clean on the removable part. I don't have any contact cleaner handy for the back plate but I could get some??

I don't know how the system would be described. Heat is gas only not heat pump backup. There is no outside temp sensor connected.

The thermostat is about 4 years old. Do I need to replace?

"I have a gas pack/heat pump that is controlled by a Honeywell."

Your description, not mine.

I would replace thermostat.

IMO

For those interested, I bought a Nest generation 1 off ebay new to replace the thermostat. While I was changing stats I discovered that the W wire was partially broken and it snapped as I moved it. That was the issue, not the thermostat. Annoying but not a terrible outcome...