Goodman MSG-12CRN1N Service Instructions Manual page 22

TROUBLESHOOTING
Diagnostic Chart
If, after energizing the unit, no indicator is lit and the unit cannot be operated, see the following chart.
Be sure all the wires (including power wires and connection wires)
Check for AC (208-230V) power supply outputs to outdoor
Check if AC (208-230V) exists at primary coil
of outdoor (and indoor) of transformer
Check if AC 12V exists at secondary coil
in the outdoor control board
Take off the secondary plug of transformer, and then check if AC
12V exists at secondary coil ofoutdoor (and indoor) transformer
Outdoor (or indoor) transformer is defective
22
(for 18k & 24k Models)
are OK
Yes
control board
Yes
Yes
No
No
No
Install good ones
No
Outdoor (or indoor)
control board
is Defective
Yes
Outdoor (or indoor)
control board
is Defective