Trying to fix a 10yo GMT090-4 gas furnace here. It's been failing to ignite intermittently for a couple of months, but a power cycle or three "fixed" it until last week. Now it seldom ignites, and when it does it won't stay on for very long. Good thing we are having a really mild winter here. The unit is blinking once which indicates ignition failure. ID blower sounds kinda tired but still activates the pressure switch every time. Water heater works as usual so I'm assuming gas supply is OK. What I'm finding though when taking ACV on the gas valve is that it's showing nothing when the valve is ON. But when I turn the valve OFF I get a 24V reading on it every time after the blower and HSI come on. Turn the valve back on and the reading goes back to zero... This is not normal is it? Would this seem like a bad gas valve?

I have lost heat 3 times this winter. The first time I had the board replaced and the Hot surface ignitor replaced and got heat for about 3 weeks. The second time I had been hearing this squeeking noise coming from the inducer motor and replaced that. I got heat for another couple of weeks. Now when I turn on the heat the inducer motor comes on and instead of hearing a click and the hot surface ignitor comming on I get 3 flashes from the green LED indicating a internal error. I was hoping some one might be able to help me as I am not sure what is causing the internal error and have basically replaced everythign in the furnace now.