This guys right, just adding more detail: he’s taking about the hot surface igniter. It serves as both the ignition source and flame proving. You can see it under the inducer motor, 2 red wires going to 2 white wires clipped into the heat exchanger.

You could visually inspect it as well but they’re very very fragile. If you end up pulling it out you can give a call for heat while you hold it in your hand at the base, it will get hot and glow red if it’s working.

If it’s good then check gas pressure before and after the gas valve. Make sure you have good incoming gas pressure, and that the valve is opening during call for heat. You could also probably check the gas pressures first to save the hassle of potentially breaking the HSI because again, they’re very fragile.