Goodman controller board fuse blows
I have a Goodman GMPN120-5, service guy replaced PCB in Aug due to A/C failure, root cause unkknown or not documented.
Last week I turned on heat for first time this season, now nothing runs. Service guy returned, located blown 3A PCB fuse, replaced, still blows. Replaced fuse with 5A for testing (see sequence below).
Main fan and combustion blower both work normally. Pressure sw works OK testing suck/blow on tube. All flame rollover sw's in NC position. 24V normal.
Disconnected all tstat wires, removed stack overht sw from circuit, jumpered R+W at PCB to get 'always ON' demand for heat.
Sequence :
1. Switch on 120V power to unit
2. combustion blower starts immediately, runs normally
3. after several secs HSI relay clicks, after several more secs HSI glows brightly
4. several more secs, relay chatters rapidly (I assume it's the gas valve relay), followed in ~0.5 sec by fuse blows
(If I turn OFF 120V pwr quickly enough during chatter, I can save the fuse and repeat sequence 1-4.)
Removed leads from gas valve, checked terminals at gas valve with ohmmeter, neither grounded.
Leave gas valve leads disconnected, repeat steps 1-4, same result.
Low V connections to outdoor unit are also disconnected for testing.
Controller board was replaced second time by service tech.
Another bad controller board?
Thanks in advance for any suggestions.