Heil NUGK075AG01
Ser:H542 11370

Furnace starts fine then after a cycle the burners won't light but the pilot stays lit without the igniters constantly firing. Blows cold air does not shut off. Draft inducer works and main blower stays on. Pilot is blue with a bit of orange but flame sensor recognizes flame and does not keep firing the igniter like it would if it were a pilot issue.

I replaced the limit switch last fall and it ran all winter just fine. I replace the limit switch because it would not shut off last year unless you banged the switch. Different problem that was solved by new switch. I bumped it this time with no luck. Limit was set at 90 deg. Drain is clear.

It sounds like you have a limit switch open. The system will have a few that can keep the heat from working. Roll out limit, high temp limit or just the pressure switch not closing or opening when it should. The fault light blinking on the control board will help narrow the search.

Unfortunately it is too old for lights on boards. I don't. Believe it is the rollout sensor. If I pull a lead off that one the gas valve shuts off the pilot as well. Still at a loss. The Fan/Limit switch is a FALTS57C-65T-120A. If it is the limit I don't understand why it works after sitting for an hour or so. Wouldn't it fail completely?

You stated that the fan runs all the time, indicating that the fan thinks the furnace upper heat exchanger switch is over 130F. The furnace might think that the heat exchanger is over 165F and shut off the main burner when that limit switch or the wiring to that limit switch is open.

I found a similar problem on a Carrier furnace where the connection from the wiring harness to the circuit board was loose, and not making a good connection to the circuit board, so the fan stayed on (thinking the over heat limit switch was open (in reality it was a loose connection at the circuit board))...

You really need to check things out with a volt meter on this furnace.

Fred.

The Camstat is actually a FALTS57C-05T-120A.

I watched it today. It cycled fine twice then when it came time for the burners to fire a third time the pilot only fired and stayed lit. I turned up the fan off switch to 105 degrees and it cut out the fan. It eventually kicks in and out from the pilot heat, but the main burners are not firing at all after the first two cycles. The Camstat is less than a year old. I am not sure if there is a warranty or not.