I had a problem with what I thought was a sticking gas valve. It turned out it was a working igniter that wasn't hot enough to light the flame. I understand the basic chain of events, but wanted to clarify the various stages of protection when/if something goes wrong.
During the time flue exhaust blower builds up pressure and the burner actually ignites, on this furnace (Rheem 90plus model from 1994 no flame or igniter sensors that I could find), doesn't the system confirm the igniter is actually glowing before it energizes the solenoid for the gas valve? If there is no sensor for the igniter, how does the system know it is working? Why energize the solenoid if the igniter isn't hot enough?
What was happening was the igniter would glow, I would hear the solenoid energize, then de-energize within 5 seconds with no igniting of the gas. I assumed the gas valve was sticking since by taping on the valve (not with a hammer) after a couple of tries, the flame would ignite. The furnace would be fine for a day or so, then fail again. I had a service guy out and we found it was the igniter even though it was working. By removing the existing igniter and comparing that with the 'glow' from my original igniter (that I saved when it was replaced), the replacement had less of a 'glow'.
What I don't understand why taping on the valve caused the gas to fire if it was the igniter.
Hope that made sense.