customer complained house 60 degrees and furnace doing nothing. I asked if she had gas kitchen stove, lit 4 burners, great flame so no pressure problems.
Gas furnace vertical, 3 gas burners 70k btu, with glow bar ignition first burner, ignition sensor on 3rd burner. First burner came on 2 seconds when glow bar got cherry red then gas valve shut off by computer board because sensor did not detect flame in third burner. Second and third burner failed to ignite.
I replaced sensor, igniter, and gas valve....same thing. So i replaced pc board same thing.
I had to mark all 3 burners so they could be reinstalled in same order. Removed burners, used very thin putty knife to clean burner pilot slots, wire brushed them and also the burner nozzles, used plastic tie to make sure no obstructions in pilot slots, and blew out with compressed air. Removed orifices and blew them out. Put everything back together and it all worked great. Hope this helps someone in the future with the same problem. Note: Be very careful of the glow bar igniter it will break "extremely " easy and it is a show stopper if no body is open (holiday or weekend) to sell you a new one.
Don't forget to bubble test all gas joints when finished.