[mjrps]

I have a 16 year old Carrier 58SXA furnace that has had problems igniting on colder and damp days for many years. Pilot, lockout device and board, fan board, and thermostat have been replaced, to no avail. Simply resetting main power supply to furnace temporarily solves problem. Pilot ignites well, then burner ignites, but afterwards on long temperature set-back delays, or periods in between calls for heat, pilot ignites, but no gas ignition. Everything works well above 45-50 degrees outside air temperature. Constant problems when outside temperature dips below 20 degrees. Could the internal gas valve heat sensor from pilot light bi-metal sensor be faulty?

[acetc]

Check to be sure you have a good ground from the burners to the gas valve, There should be a green wire attached to the burner manifold.

[mjrps]

I don't see any green wire attached to the burner enclosure or manifold, and I don't see where one is called for on the wiring diagram. The gas valve appears to be well grounded mechanically by the gas pipe and brackets. The inducer motor and bracket are grounded by a green wire. I didn't mention in my initial inquiry that the pilot won't light after a recent burn, even when the heat system is turned off and on by the thermostat or high voltage supply. I have to wait quite a while, before resetting the system or power works. During that time, the gas valve just seems to click away. When I do wait, resetting the system or power seems to work 100%, but only for one burn cycle.

[acetc]

Do you have a pilot with three wires ? You mention bimetal so that may be where your problem is the three wire pilot has a warp switch that when it gets hot the bimetal bar moves over and makes a set of contacts in it to bring on the gas valve,(must have a good blue flame) these contacts get dirty and don't make always. When it fails to work tap on it with a screw driver handle and see it it comes on, or you can varify it with a volt meter at ythe gas valve terminals.

[mjrps]

It is a three wire, but I don't think it's the problem. When the unit doesn't want to start, the gas valve clicks, but the pilot isn't getting an electric spark to start it.

[mjrps]

Also, the pilot flame sensor is a 3 wire assembly that has a metal or bimetal strip folded over at the rear, and when hot, apparently forces a plastic plunger into spring loaded copper wafer elements with a couple of points for contacts, atached to a ceramic switch/wire base.