Keeprite NG furnace code "fail to ignite"

Good info, thanks!

So it's not your stat, or wiring....

So back to gas and voltage. Are you absolutely certain you have gas at the furnace? Also, have you checked voltage at the MV terminal on the gas valve?

Your HSI looks good (usually they either work, or they fail completely), and based on experience (in my opinion) I feel it should light-off the gas, but I've been wrong before. (And who knows? Maybe your gas supplier has an additive that has raised the ignition point of your NG, I don't know.) Thankfully the P/N on that plug is used by many other manufacturers, and it's a pretty cheap part, so as much as I hate "parts swapping", if you can locate a new one, by all means- have at it!

Last thing I can think of- maybe your gas valve is making, but you're not getting any gas output (bad gas valve). This is easy to measure if you have the right tools (pressure S/B around 3.5" W.C. on the gas manifold), but not too many people own a gas manometer.