Furnace / AC problem?

I replaced the thermostat in my 5er and discovered that the problem was actually NOT the thermostat. We had typical thermostat problems too, like staying off too long, not shutting off at the set temperature, etc. I have since learned that for most 5er's, the furnace is designed to run off 12 v batteries, so they are much more sensitive to workload and maintenance than house furnaces. Sometimes little things like dust buildup or debris in the unit can cause it to lock up. If your problems are not fixed when you replace the thermostat, I would start by doing very extensive cleaning of the unit/motor and see if that makes a difference.