I have a 3 yr old - 2 stage American Standard gas furnace. I tried to use a generator in an emergency, by disconnecting the hard wiring from the switch next to the furnace - and putting a good, 3 prong pigtail on the wires coming from the furnace.
All I got was clicking and a sound like it was 'trying' to light and a really cold night in the house.

It has been suggested that while the generator was big enough and the cords were big enough - the ground might have been insufficient. When I unhooked the white and ground from the house wires I was relying on the generator ground to complete the circuit? Does this make sense? Can poor grounding be my problem? When I reconnected to the house wires and the power was restored - everything worked great. The furnace is on its own 15 amp circuit...
Anybody out there who can help with this one.. :o

See https://www.askmehelpdesk.com/electrical-lighting/generator-bonded-neutral-55551.html

I have read some other reports here on furnaces refusing to run on a generator. Not fully sure what the problem is. I am concerned because I have a generator that I would want to use for that.

I have friends who use generators to run their gas furnaces in emergencies, by rewiring the disconnect power switch to a short pigtail male plug and then using a good extension cord.
Theirs are all at least 10 yr old furnaces and all worked fine. I tried it with a newer model(3 yrs old) and it won't light. I admit that I probably didn't have an effective ground system after I disconnected the white from the furnace/house and hooked it to the pigtail's white wire - - same thing for the green/bare ground.
Others who had problems with furnaces may want to think about their hook-up and see if they may have done the same thing.
I plan on testing the connections again soon, with better ground connections.
Anyone who can add to this discussion - please do. I understand the emphasis on transfer switches, but - I'm using cords.
Can somebody answer this - - simply, for us thickheaded readers? ;)

I see on your wiring digram you sent you have to have an equipment ground on the gas valve if you do not have a good ground it will not work. the flame sensor can not sense the flame if the valve is not grounded.

Interesting question. This came up in a thread on a furnace that won't run on house power. I wonder if that is why some people have trouble getting their furnace to run off a generator. If the frame of the furnace is properly connected to the supply ground wire, and they use a good 3 wire pigtail, I don't understand why it wouldn't work. If whoever installed the furnace skipped or botched connecting the ground wire, it might, gack, ground through gas line. Then not be grounded to the generator.

After an ice storm here in Ohio, we had several customers running their units on generators.

A few had trouble and I was called out, I found 1 needed an earth ground to work properly(flame sensing issues) and another that needed 60 cycles per second before the board would activate. The second one we adjusted the cycle rate and got temporary heat.

Another customer had run out of fuel and when he restarted his generator the control board failed, I'm guessing a brownout condition as the generator was stalling due to lack of fuel.

As I indicated - I would admit I had the grounding bad. I had not grounded the generator - and I had removed the furnace from the house ground before I hooked it to the pigtail...
I could hear the igniter, but as you noted - the gas valve may not have been sensing and or calling for flame.. .
I hope my mistake makes it easier for the next guy. It sounds like there may be others out there wondering "why..?"
I appreciate the informed response from an HVAC guy, whom it seems has worked through this for someone else. I know the storms will continue, this time I think I might have it right...