Goodman GMPNO-80-4 Limit switch code

[laxdef69]

When it kicked on this morning, there was a VERY loud humming noise, as if a motor were straining...my wife asked if we were about to have an explosion, thats how loud it was.

I checked both the blower and the draft inducer, and they spin freely.

The status light on the board flashed four times, which according to the chart on the blower door suggests either a main or auxilary limit switch issue.

I have found a limit switch, but not a second one.

Unfortunately, I have no idea what to test for...resistance, continuity, etc...or if a bad limit switch would cause this humming noise; The capacitor tested out fine.

I bypassed the door switch and turned up the T-stat.

The draft inducer motor runs quietly, and induces the draft;  the heating element comes on, the burners fire for about 20-30 seconds, then the loud humming starts...sometimes the blower spins, sometimes not.  It seems to get loud about every 4th or 5th cycle, then sit there like a petulant child until I turn off the power to the unit for a minute or so...

I don't know enough about electric motors to know if this one has ports to oil it...

Thanks for your help, oh wise ones!

--------------EDIT-----------------

I pulled the motor and had a local HVAC shop test it...it had a bad winding, which was the straining noise.  The new motor is nice and quiet!

[hvacdrd]

Job well done. That explains the limit fault code.