Hello,
First timer here. Didn't see my 1st question post, so I'll try again. About 20-22 years ago, I installed a Carrier 80% furnace. I did do it myself, but only have basic HVAC knowledge, however it has been working fine until recently. Now I am getting inconsistent lighting of the main burner. Digital thermostat is set for 70 degrees, house temp is 62. Pilot igniter comes on, pilot ignites, burns for 3 to 4 minutes, then goes out with no main burner ignition. I have turned the furnace power off/on a couple times, and same thing happens. Removed lower panel, operate safety pressure switch myself, and usually get same result. However, after replacing lower panel, pilot came back on and main burners ignited, followed shortly by blower & heat. I have tried this same procedure before, removing & replacing lower panel several time without success before, only to have the furnace come on normally on its own some time later. What could be causing this inconsistent operation.
Thank you,
Chilly in Pittsburgh

Moving the panel and banging it around might be a indication of a loose or corroded wire somewhere in the furnace.

Model number?

It is a Carrier 58GS100-GA, series 160. I guess one of the problems here is the inconsistency of the problem. Again, this morning, the furnace failed to come on when the temperature dropped well below the thermostat setting. However, this time, I turned off the main power to the furnace, waited a few seconds, then back on. On the first attempt, again, the pilot light came on, burned for a few minutes and shut off. On the second try however, the main burners did come on and the furnace ran through a normal cycle. About 1 1/2 hours later, it came on again on its own, and again ran through a normal cycle.
The good news here is that we probably won't need the furnace much more this year, but I'd rather figure out the problem know than wait till next fall, when we really will need it. Also, I wondered if this will effect the air conditioner system. Obviously, no pilot, no gas, but some of the same circuitry?

Does this sound like your problem?

QUESTION? Does your pilot have 3 wires coming out the bottom of the pilot itself?

What I have found on your furnace so far is conflicting, but the best I can tell is that it has a circuit board for controlling the fan, and a spark ignition to light the pilot and then the burners. I think that there was also a pilot relay style pilot burner (740 style) with 3 wires - white, yellow, and green.
I know that the original circuit board was updated a number of years ago. If you circuit board has two clear relays on it then it is the old style. The newer board has 2 black relays.

I am not sure what turning the power off then back on to your furnace is resetting because the original spark box for that furnace was not a lockout type. Cycling off the power normally only resets lockout type spark ignitions. It would appear that your ignition may have been changed from the original.

Lockouts happen on older spark ignitions when they try for 5 minutes to light without proof of flame. This means that the lockout could be happening because the pilot is not lighting or it could be happening because the pilot is lit, but the flame sensor is not responding.

There are so many different variables as to what could be causing this that I can only tell you what I would check. I would reset the lockout and then place the system into a call for heat and watch what happens. If it is trying to spark, but not igniting, then there could be a problem with the pilot gas flow, distance from ignitor to the pilot hood, the gas valve itself, or the strength of the spark. If it lights, but doesn't respond properly then there is a problem with the new 740 pilot burner.

What this requires is a person who knows what to look for, has the meters to read what is happening with the voltages, and the patience to wait for the problem to duplicate. If you can do this, get back to me and I will try to find a wiring diagram so that I can tell you where to test.

OK, here is what I could find. Switch of pilot burner does have 3 wires, white, yellow & green.
Circuit board only appears to have 2 wires going to it. One brown, then a Red wire with what appears to be some type of resistor between the end of the wire and the board.
There is also a large gray box that says HSC Igniter/Lockout, (1007-100, LH33WZ512A, 9092). On the circuit board, I could see these numbers, 36T01B3, L250-40F, HH12ZA251A & J9030, MEXICO.
Here again is what has been happening. Thermostat calls for heat. Igniter sparks, pilot ignites, burns for 3-5 minutes, tip of what appears to be thermocouple glows red, but the main burners never come on.
I also found this while poking around. There are 2 wires attached to what I think is the fresh air sensor on a panel just above all the controls. One of those wires fell off when I was cleaning out dust. I tightened the clip & replaced it. This just happened, and the temperature has warmed up here, so there hasn't been a call for heat.
Nothing has been changed on the furnace since I installed it around 1986 or 87. Whole house AC was added about 4 years ago, by a qualified HVAC guy, but to my knowledge, he made no changes in the furnace circuitry.

I would say from what you mentioned in your first post that the 3 wire pilot sensor has become intermittent. It is a wonder it has lasted so long.

If it looks like the picture this is it.

LH680005 CARRIER/BRYANT/PAYNE 3-WIRE FURNACE PILOT - eBay (item 140232084284 end time May-17-08 20:30:05 PDT) (http://cgi.ebay.com/LH680005-CARRIER-BRYANT-PAYNE-3-WIRE-FURNACE-PILOT_W0QQitemZ140232084284QQihZ004QQcategoryZ4198 7QQssPageNameZWDVWQQrdZ1QQcmdZViewItem)



NOTE spray some WD 40 on the aluminum tube at the brass nuts. If the aluminum tube is corroded inside the nut you will be buying some new tube and nuts if you do not lubricate that area well before disassembly

YEP,That's it. Thank you for your help, and also the eBay site. I was going to ask you where to buy it. I saved the eBay site, and the repair is now on my "to do list". I am just curious about one thing. Since the pilot light ignites perfectly every time, why is this item the problem? Does it send a signal to the gas valve that it is OK to open up or what?
Thanks again,
Warming up in Pittsburgh
GO PENNS!!

A 3 wire pilot has a proving safety switch built in it. So the furnace knows that there is a pilot lite lit and it is OK to turn on the main gas. This is a simple explanation.