1. the t stat calls for heat
2 usually the inducer mtr comes on next
3this has to PROVe. by either a pressure switch or sometimes a centrifigal switch
4 after that, then usually the ignitor comes on
5 then after it is hot enough, the gas valve opens.
so.. if the ignitor does come on and glows etc.. the gas valve "should" come on. do you have a meter by chance? if you do like DIY's you should get one if you don't already. I have bought cheap ones before, kinda spur of the moment needed 1, and mine wasn't around.. for 20 bucks and they still work fine! IF YOU GET 24 VOLTS AT THE GAS VALVE AND IT DOESN'T OPEN ( FIRE) THEN IT IS THE GAS VALVE. IF YOU DON'T GET 24 VOLTS, IT IS IN A CONTROL. whoops! just noticed I hit caps! sometimes those ignitors LOOK hot enough, but are not hot enough to PROVE they are on.. if you replace that ignitor, do not touch it( the end) (treat it like a halogen bulb)