I don't normally work with funaces though I do understand, for the most part, how they work. I'm a bit perplexed about a problem I'm having with the FG6 as I'm not seeing what I'd expect.
The firing sequence makes it all the way to the hot surface ignitor glowing but there's no gas. After a while the ignitor goes out but the controller never trips out - it just keeps cycling between venting and ignition.
My first thought was bad gas valve as it shouldn't be powering the ignitor unless all the safeties are all happy. I disconnected the valve power and hooked up my meter. As expected the furnace tripped saying the gas valve was bad but what I didn't expect to see was a constant 14V up until the trip. I thought to see 0V until after the ignitor was glowing brightly then see 24V.
I checked the thermostat terminals and those have 24V so I assume the transformer is good.
I expect tomorrow I'll have to start checking the control voltage at each safety (had to go out so couldn't do it today) to see if one of them is to blame or if the control board is only supplying 14V somehow.