Thanks in advance for any assistance you can give me..

I have a 1991 Tempstar model 9580GF 90% efficient upflow condensing natural gas furnace. The long model number is NUGK100DH08, doubt that helps. A bit of background, I recently (3 weeks ago) replaced the combustion blower as it had gone bad. Note: it sounds like a lot of condensate is moving through the blower but it seems to be draining normally and flue is not plugged up, seems to vent correctly. Furnace ran fine.

Awoke this morning to no heat, could hear the pilot ignitor cycling. I turned down the thermostat, turned off the power at the furnace switch, turned off the main gas valve, and turned the Honeywell gas valve to off position to reset the system. After 10 min. I turned everything back on and here is what happened:

1. The combustion air blower turned on normally 2. The ignitor "buzzed" about 10 seconds and what appeared to be a "good" pilot lit 3. The main burners then lit up normally, good soft upward blue flames. 4. With main burners still on the ignitor "buzzed" several more times as if still trying to lite the pilot light... appears to not know the mains are on. 5. Mains stayed lit about 30-45 seconds then went out, ignitor still cycling and pilot still lit, the mains almost lit again but didn't. Tried again same thing. Ideas? Flame sensor?

That would be my guess. I don't like to throw parts at something, but it is hard to check out things in modern furnaces. It is cheaper than a control board.

Thanks a lot labman... will give that a try.

Yes it sounds like a dirty flame sensor. Carefully remove it so as not to break the ceramic it is in and clean it with some crumpled up aluminum foil or fine steel wool

Thank you NorthernHeat:) (you too labman!) Are flame sensors available at an HVAC distributor only or do you think think a hardware store might carry new ones?

Just to follow up--cleaned the flame sensor with crumpled aluminum foil.. worked like a charm! Wife and kids warm and happy now.. thanks again.

GorDog