On Thursday, October 26, 2017 at 3:58:01 PM UTC-7, wrote:
Hi All ; just a heads up - learn from my mistake.
I just had to replace my 8 year old thermostat -
- the touch-screen was failing to adjust certain settings -
including the set-temperarture - it was stuck on 64 F. degrees.
I discovered the 3 AAA batteries had corrosion - no doubt
after being neglected for 8 years ! The other little 3 volt battery
that was well-hidden - seemed fine. I replaced it in an attempt to
reset / revive the unit - to no avail.
I'm not positive that the cause was the batteries but I can't
think of anything else to blame ..

https://yourhome.honeywell.com/en/pr...ostat-rth8500d

John T.

It was the batteries but you got a good 8 years out of it.