I was online last May and "hvac1000" was very helpful in helping me solve a problem, but now I have a similar but different problem.
Last May, my furnace, (Carrier 58GS100-GA Series 160, about 1986/87 vintage), wasn't lighting properly. The pilot relay burner, (740 Style) would spark, pilot would light, but main burner failed to come on, thus no heat. Hvac1000 not only suggested replacing the 740 style relay, (a 3 wire, white, yellow & green wire, LH680005), but he even directed me to an eBay site where I could get it. I purchased the relay, installed it, and things worked great. However this season, similar but different problem.
New problem: Apparently furnace circuitry isn't picking up the call for heat. Desired temp can be set for 70, but even after actual temp dips to 65 or lower, furnace doesn't come on. However, if I go down and turn off main power to furnace, wait a few seconds, then turn it back on, everything seems to work fine. Pilot sparks, pilot ignites, after a few seconds, you hear main gas valve open, main burners ignite, and furnace runs a normal cycle. Sometimes the furnace will run 2 or 3 normal cycles on its own, and sometimes it won't. So far, every time I have used the method of turning off the power to the furnace and back on again, the furnace has worked. I've noticed a few times that it took several seconds, 30 or more, for the pilot to ignite after the sparking begins, and some times it lights in only 4 or 5 seconds. I use a 4 wire, programmable thermostat. It also is fairly old, but it worked fine all summer with the AC.
At first, this problem only occurred very infrequently, and usually after the furnace had been off for some time, with no call for heat. Now it seems to be happening once or twice a day. Since we both work, and are gone most of the day, it is hard to tell if it comes on by itself at all during the day. The dog won't tell us.
Please help.

First here is a startup and check out manual that will contain some info for you.

http://www.xpedio.carrier.com/idc/groups/public/documents/techlit/58gsc-5si.pdf

You have a problem that servicemen hate since it is intermittent and that means that the defect be it a part or a loose connection only happens once in a while and not all the time.

I would start by disconnecting the W and R wires at the terminal strip in the unit. Make sure you have the wires labeled or if the color matches W to white and R to red it explains itself for reconnect purposes. I would attach two wires to the terminal strip as a thermostat bypass. Test the furnace by connecting the wires and allowing the furnace to come on. Wait a minute then disconnect the wires. Do this 5 or 6 times and see if the furnace comes on every time. If the furnace does not come on every time notate that and post back with your findings.

This is a very simple thermostat bypass test. If the furnace works every time the wires are well connected it means that the problem could be in the thermostat. Normally thermostats do not cause this type of problem but this is a easy est for you to do.

Next test will be on the door switch test but report back on the thermostat test first.

HVAC1000,
I'm back. This goes back to your response to my last post on 11/18/08. You instructed me to do a thermostat bypass test. Your test was easy to understand, and I was able to complete it. I connected and disconnected the two jumper wires eight times. Each time, letting the furnace come on all the way into the blower cycle. When I disconnected the two wires, first of course, the burner shut off, followed shortly by the blower. The furnace seemed to light perfectly each time. I have since reconnected the regular red & white thermostat wires, set the desired temp up 5degrees above actual temp, turned power to furnace back on, and furnace came on and ran a normal cycle.

Since my last post on the 18th, the furnace seems to have been working fine until today. The temperatures have warmed up slightly, and there hasn't been a "call for heat" for several hours. However, when inside temp dropped to 65 & thermostat was set for 69, I noticed the furnace hadn't come on. I went down and tried the old method of turning off main power, but when I turned it back on, the igniter sparked for almost two full minutes, without the pilot ever coming on. That's when I got out your thermostat test suggestion and did the test. Also, I travel a lot, and have been out of town much of the time since the 18th, but wife said she thought it was working OK while I was gone.

Maybe its just allergic to me. Should I just try replacing the thermostat? The only unusual thing I have noticed is that when the problem occurs, and I turn the main power to the furnace off & back on again, it seems like the igniter sparks for an unusually long time, before the pilot finally ignites and the burners come on. As I mentioned, this last time I tried this method, the pilot never came on, and the ignitor eventually stopped.

Any more suggestions.
Thank you

Thats when I got out your thermostat test suggestion and did the test.

When you did the test this time did the unit work normally ?

If it worked when you eliminated the thermostat this time then I would suspect the thermostat is defective. It is also a REMOTE possibility that the thermostat wire is also causing the problem. Post back and let me know if when you do the bypass if the furnace comes on and off normally.

Yes. When I did the bypass, the furnace came on, ran normally and when I disconnected the two bypass wires, the burners shut off, followed shortly by the blower. I did the test 8 times. Perfect all 8 times. When I reconnected the thermostat wires, the furnace did come on and ran at least two complete cycles on its own, however, it failed to come on for the morning warm-up cycle, and failed to come on all day today. When I got home, house was 56degrees. Burrrrr. I had left at 4:30am, and when my wife got up, she tried to get it to come on, using the power off/power on technique that has worked in the past. She said pilot would spark for about 2 minutes, but pilot never lit. I tried the same thing when I got home at 6:30pm, and pilot lit right away, followed by furnace.
I'm on my way to Home Depot to get another thermostat. Keeping my fingers crossed. Is it possible that the ignition switch I purchased in May was bad or has gone bad. That's what you recommended back then. We really didn't have much cold weather to give it a run for its money back then.

To easy your flame sensor in going and either needs to be cleaned or replaced .also check ground to earth possibly wired to main from switch with no ground causing a lack of circuit for flame sensor .flame sensor is located in the flame spread a little rod straight or l shaped try cleaning with sandpaper.

Sorry I didn't read year of make is spark ignitor after market

If the unit works when you do the test the furnace is OK

The problem would be isloated to the thermostat wiring OR the thermostat itself.

Post back with results.

hvac1000,
Thanks for all your help. Installed a new thermostat, and system seems to be working fine. Don't know your name, but really want to say thanks for your patience and clearly thought out suggestions. The thermo bypass test really spelled things out. I wish I had replaced it right after I did the test, then I wouldn't have come home to a very cold house. Any way thanks again, and I hope you don't hear from me again anytime soon.

Good to hear you got it running.