furnace won't stay lit

Old 12-26-12, 11:28 AM
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furnace won't stay lit

OK light...single blink after 4 or so start attempts (soft lockout)
MFG : 02/2006

Cleaned flame sensors, blew out drain lines, ensured hoses between two shutoff switches were not obstructed, placed paper clip into both as well; ensured outside vent was clear.

Two switched part numbers:


It's snowing, cold, and after recycling power, unit will run for about a minute, flames ignite, blower motors run, then flames go out, and blower motor runs for about another minute before shutting down. This cycle repeats 3-4 times, and that is is. I can't get the house above 67 degrees or above the desired temp. I do get the "soft-click" in the thermostat when moving desired temp above room temp.

I couldn't get anyone out today if I tried. Roads too bad. Thanks for anyone out there able to help me troubleshoot this bad boy.

Old 12-26-12, 12:03 PM
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And on iterations there is some mechanical switch-sounding clicking that occurs @ unit, with the flames going out with blower continuing to run for a minute or so but only blowing air, not warm air as the burners are out. it then relights after a bit and the cycle continues....until, I believe, it hits the 3rd or 4th attempt and goes to the soft-lockout. I've also replaced the furnace air filter, and the energy saving thermostat is about 2 years old...good batteries. Thanks for replying!
Old 12-26-12, 09:33 PM
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Not sure if that unit uses a combination fan thermostat and hi limit switch but from your description of the problem it does. The unit would make sort of a loud clink sound and shut down the burner. After it cooled down the burner would relight.

This switch would shut down on overheat if you had restricted airflow thru the heat exchanger. Possibly a clogged air filter.

This is the inside view. Would have a metal cover over it.
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Old 12-27-12, 12:46 PM
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Odds are you are describing a furnace that is shutting off on the high temperature limit switch because it's overheating.

Use a multimeter to see if the high temperature limit switch is what is shutting off the burners.

The most common cause of this problem is something that is im[paring air flow through the furnace, such as plugged filters, a cold air return that is obstructed or too many warm air vents that are closed.

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