My Goodman furnace sometimes starts up, vent fan, then the gas does not ignite or even appear, then blower runs through its cycle and shuts off, blowing unheated air around. It does this about 30% of the time now. A HVAC pal filed off the igniter rod saying it was corroded. He said there was something that needed to be adjusted as Goodman did not make a certain valve for higher altitudes. I am at 5300 feet. Any ideas why it is not going flame on?