Question– “My heater stops kicking on. I turn the thermostat to “off” & the heater kicks on. I turn it back to “heat” & it turns back off for a couple of seconds, then kicks back on & runs fine for a few hours. The local repairman changed out a circuit board, then said I should replace the thermostat, which I did, but it’s still acting up. Unit is a Rheem Criterion II gas furnace.”

Answer– In our opinion, sounds like the flame sensor or the fan limit switch needs to be replaced. The fan limit switch turns the gas valve on and off regulating the temperature inside the furnace, if this part is malfunctioning the blower will continue to run without the gas valve opening to turn the flame on. If it is a bad flame sensor the flame will come on and stay on for only about fifteen seconds and then abruptly go out. Both parts are less than $15 each.