I have a 2-wire (red and white ) coleman manufactured home thermostat that sticks and I've tried to replace it to no avail. Replacement works till I try to switch the unit over to air then it runs constantly(as if in "fan only" mode)
How do I replace a 2-wire heat and air system thermostat?

I've only encountered one of these 2 wire heat/cool systems in the 30 years I've been in the trade. If I recall correctly, I had to go back with the same type of thermostat as originally installed.