Gas and Oil Home Heating Furnaces - Trane Xl-80 won't fire

I have replaced the ignitor and the gas valve and still won't fire. The ignitor glows red and the gas valve will open for about 2 seconds and then closes before it fires.

Sounds simply like you need to steel wool clean the flame sensor rod. If you have a flame sensor, it will be near the ignitor and usually you can see one skinny white wire leading to it. The ignitor has generally 2 white wires going to it, and if yo see this extra white wire heading down into the same general area in your burner area, that would be the flame sensor. They are usually held in by 1 screw and are usually relatively easy to get to and to remove. Then just polish the rod with steel wool or fine emory or sandpaper.

Sorry to hear that you replaced the expensive gas valve most likely for nothing.

I just thought of something else that does not apply in your case but could apply in other people's cases that have similar symptoms as yours and they HAVE polished the flame sensor. It is possible that an ignitor glows orange but is not putting out quite enough heat and will not fire of the gas. Then, the furnace senses this and it shuts down. In this case, as crazy as this seems, one needs another ignitor.