I have a Tempstar NTGM075EGA3 furnace, about 7 years old, when thermostat calls for heat the igniter comes on, pilot lights, and burners come on for about 3 seconds and go out. Pilot stays on, blower comes on and stays on. If I turn furnace off at thermostat and let blower shut off, most times it will restart and run fine. Sometimes I have to tap the Honeywell Dumbvalve to get it to work. The HVAC tech has been here twice and was not sure but suspected the valve so I replaced with a guaranteed-to-work rebuilt one. Fired up and ran fine but tripped overtemp. Do I need to buy a new valve?

Or a new furnace>lol. Does your furnace have a flame senser? If so, clean or replace it. Cleaning is just as good as replacing. If you could take some pics of your burner area, I could tell you if you have a flame senser or not.

I have a flame sensor. This the third. The pilot lights, the flame sensor tells the valve to open, the burners light and stay on for a few seconds then shut off but the pilot is still on.

Okay. Lets start fresh. You said burners would come on for three seconds. You replaced valve(used or otherwise). Then burners stayed on long enough to open high limit? Or are you still getting only three seconds of burner time? If burners stay on now until high limit is tripped, then it sounds like you may have an airflow problem(or possibly a bad limit, not likely) Was filter neglected for a long period. If so, you may have a clogged condensing coil in the furnace (located directly above blower assembly) Or a clogged evap coil if you have central air. Please let me know if your airflow has slowed considerably. This would cause high limit to fail. Hoping you don't need a new gas valve. Were did you get the "rebuilt" one? Ive never heard of a rebuilt gas valve. If valve wasn't purchased from a reputable company, or a manufacturer, then the valve probably never should have been installed in the first place. Just a thought. Please let me know what you find.

Heh heh... you said "dumb valve"... how right you are sir! I would bet the farm that is where your problem lies. Try applying pressure to the square connector on the valve. If this affects the function of the unit (i.e. it starts working), replace the D.V. P.S. Make sure the replacement has a date code of 2007 or newer to assure you get one of the "improved" versions.