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Is this a GMP or GMPN?
in the same series as the gas valve....meaning once power is sent to gas valve it must cross a couple "bridges"...
Do you know if your unit has an auxiliary limit? If you trace the 24v wire backwards from the gas valve does it go to a limit in the blower section?
If no 24 on 3 and 6 then the transformer is bad. This can be because of any short in the low voltage circuit.
A transformer can run hot with a partial short to ground, bad gas valve etc. After cooling down it can work until eventually burns out. Is there a fuse on the board (yellow 5 amp)?
Some transformers you have to watch how they wire...especially multitap...we are trained to think black as power...but the transformers (some) use black as common...
If you get a multitap just watch how it is wired..