I have a Lennox G32-75 gas furnace and a Honeywell T8401C thermostat.

I have been setting the thermostat temperature at 69 and the furnace continues to heat the house higher than the preset (up to 77-80 degrees). I am having to shut the thermostat off (which does shut off the furnace) and then bring the temperature back down to 69 (or below) before I turn the thermostat back on for heat.

I have been having this problem for the last month, however when I had the thermostat off for a few hours and then started back up, the temperature would stay at 69 for a few days and then all of a sudden, start to go up again. Just in the past couple of days, this has not been working... I turn the thermostat back on at 69 and then after about 30 minutes, it starts heating the house back up over 69.

I have noticed that when the house is heating up over preset temperature of 69 (say 71) the furnace does shut off for a time, but then turns back on at 71 and heats the house more.

Before I call a technician, I want to make sure that #1) it's something that I can't fix myself and #2) If I can't fix it myself, understand what the problem is so I don't get railroaded.

Thanks in advance for your help!

The first thing I'd suspect is the thermostat as shutting it off appears to clear the problem temporarily. If you know how to use a multimeter I'd be checking what's energized on the thermostat the next time it acts up.

More than likely the problem is with your thermostat. It could be some sort of adjustment but it could also be that you need a new one.

On the other hand the start and stop of heating you mentioned if not being caused by the thermostat could also be the signs of a furnace problem.

It is almost impossible to tell from your description what is going on but I would bet it is the thermostat.