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Can you tell me the pattern of the blinking light?
Is the exhaust fan running?
So it turned on to test but not now?
Thank you....the fan I am interested in right now is the smaller fan in the upper section connected to vent pipe. Is that motor running?
Ok...now I need the model number please.
Take upper door off will be on side panel inside...something like GUI****
How about GMH?
I do not see GMH anywhere...?
I found it...The unit is made by Goodman and marketed as Amana.
Model G MH is the Goodman and A MH is Amana but they are the same thing.
The code 3 is pressure switch not closing. The pressure switch closes after the exhaust fan starts . Your exhaust fan is not starting thus the code.
You have either a bad exhaust motor, stuck exhaust motor or the circuit board that turns on the exhaust motor is bad.
If you have a test meter we can go further.
The goal at this point is to see why the exhaust is not running. You can take it apart to see if something is inside (bird, rodent etc). After that requires checking for voltage.