2000 Model UGRA-10EZAJS - furnace turns on and lights for 8 seconds - then shuts off - repeats 3 more times and then locks out - no yellow LED on the IFC board whenthe flame is lit - tried cleaning the flame sensor - tried replacing the flame sensor - checked ground continuity- checked continuity between flame sensor and IFC - checked voltages on IFC - all at 24 VAC - checked polarity of 115 VAC - checked 115 VAC to chassis ground - checked igniter - replaced igniter - checked continuity igniter to IFC - replaced IFC twice (model is a 2008 "A" version versus original 1999 "plain" - tried new 115 VAC line direct from circuit breaker box - tried grounding to burner frame - cleaned burners (not really dirty) - checked limit switches - no fault lite codes on IFC - no signs of damage - removed thermostat connections and jumpered IFC terminals - still no flame sense - ideas??
Check the inlet gas pressure to the furnace with a manometer. I had a intermittent problem once where the gas co regulator was dying. Would build up pressure and as soon as the gas valve opened would lose pressure/fade out. If you don't have a manometer get the gas co to send a tech out as it is their responsibility to provide you with a steady supply of gas. If you have propane then you need a propane tech and a manometer and check your tank/needs to be at least 1/4 full. Check for a steady 24 volts to the gas valve. The valve may be malfunctioning also. Usually they are good for about 12-13 yrs before acting up.
I am having the exact same problem as you, no flame sence at all .. I have check all the items you listed plus the gas pressure I am a trained tec on Ruud/ Rheem furnaces and have run accross this problem before.. and have always been able to get it figured out, not this time spent 5 hrs thus far and cant get. Please reply if you have figured any thing out Thanks for any hel you can offer.