Furnace not working? - It may not be the furnace!

I received a call from my tenant in the middle of the winter saying the furnace was no longer working.  I do not know anything about furnaces so I brought my father-in-law Doug with me.  Doug looked at it but could not find the issue.  Just before giving up Doug tired wiring around the control center and the furnace came blazing on.  That told us the furnace is working fine and the issue must be the thermostat!  We went to check the thermostat and we found a new digital thermostat hanging off the wall by one wire and a 2nd wire not connected at all.  The tenant attempted to install a new digital thermostat (without my permission) and didn't feel the need to bring this up during the phone call.  We correctly installed the new digital thermostat and everything was working fine.

ONE MONTH LATER...

I received a another call from my tenant saying the furnace stopped working and to please fix it right away.  The first thing I did was check the digital thermostat.  It was on but without the back-light.  I replaced the batteries and everything was working fine.

Lessons Learned: 

1)  You never know if you are getting all of the information from the tenant.  I almost spent money on a maintenance guy to come out just to tell me it was the thermostat.  Trust but verify!

2)  When the batteries are low in a digital thermostat they are not strong enough to power on the furnace.