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Goodman GMNT furnace goes into 'lock-out'.

On cold demanding days, my Goodman GMNT gas furnace operates fine for about 20 minutes, then goes into a lockout for 60 minutes (LED blinks once). If I set the thermostat low (so the unit can meet the demand) the system shuts down normal (not locked out and steady LED). The flame sensor has been cleaned (sanded, not replaced), the RO switches are OK, the vent pressure switch was replaced (mfg type and not a generic), stack over-temp switch seems good. Gas regulator and pressures seem good. What's causing the unit to go into lock-out mode (according to the mfg literature only the flame sensor 'three time' failure causes this)?

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  • stobbit Jan 09, 2009

    Two quick responses! Thanks!

    It looks like the vent intake caused the problem. All I did was take off the top panel (exposing the burners and gas valve on the upper section and the fan motor in the lower section). The panel has louvers on the lower section but none on the top half. There is only a 3" circular hole on the upper side panel to let air into the burner chamber. Evidently, with the front panel off, the burners get better ventilation and the system works properly (shut down with a steady LED). The side hole is directly beside the flame sensor, so I assume that end burner flame 'jumps' due to air intake from the hole, and does it enough (3 times) to send the unit into lock-out. I've temporarily left the panel off to make sure this is the issue. If it is, I'll cut additional vent holes in the panel to ensure adequate air flow. This system is 6 years old and I cannot understand why this condition started now....

    Thanks again!

  • samene Mar 12, 2009

    Same thing, how did the 'foor off' test work out? What was final resolution?

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The brain has to have constant verification of flame. If the burner ground is corroded then you will get the condition you prescribe.

Posted on Jan 08, 2009

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Check to see if your intake pipe or vent is clear this could cause a flame jump and make it look like a bad flame sensor let me know if this helps

Posted on Jan 08, 2009

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