Coleman 7956-856 won't ignite/run

[winter]

My husband wired a plug to our Coleman Presidential 7956 gas furnace during a power outage to run the furnace off of a small generator. The first couple times this worked fine but on the last time, in a panic, he wired it wrong and back-fed to the furnace. He turned the furnace off 'immediately' but it was too late of course. I have replaced all the switches and several transformers but not the thermostat. When the fan is turned to "ON", the blower runs fine. When the thermostat is moved up to call for heat, nothing happens but we hear a click in the thermostat. Furnace does nothing. The burner does nothing (cleaned the burner last year before this happened). Checked wires. Wires look good with diagrams. Help troubleshooting would be appreciated.

[Robert]

When you get a call for heat from t-stat, see if you get 24vac at gas valve terminals.


Thanks,
Robert