My Bryant Gas Furnace occasionally fails to ignite. When this happens, I can reset it by turning the switch by the furnace off and on. It will then ignite after sparking for 30 seconds or so. The furnace has a pilot that is on and becomes large when the thermostat directs it to, but after sparking for a while the spark shuts down if the gas valve has not been opened.

When working successfully, the sparking occurs rapidly for 10 or 20 seconds, slows down quite a bit, for 10 more seconds or so, then I hear a notice what I assume is the valve being signaled to open, then a second or two later it ignites

I do not understand

1. why there is a spark electrode when there is a pilot
2. whether there is a failure of a sensor
3. if there is a sensor or if it is part of the spark electrode
4. is there a problem with the gas valve
5. is it correct for the sparking to change from rapid sparking to slow sparking as I escribed above or whether that is an indication of the problem.
   

Some older furnaces have a pilot that may turn off each time the thermostat is satisfied and the sparker re-ignites the pilot when the thermostat calls for heat.
Some spark ignition systems have a flame sensor rod and some use the igniter spark rod as the sensor, when you look at the pilot you will see either one or two rods, each will have a wire attached, the flame as it is lit provides a ground source for the ignition control board , when the spark slows down, the flame is making a ground. The most common problem is corrosion on the rod, even a small amount will cause problems, sand the rod with fine sand paper.
If you have a three wire pilot as in a BDP furnace replacing the pilot may be your only option, these pilots have a set of contacts inside the pilot driven by a warp switch.
Good luck, Mike

Hey Mikey42, Do you have one of the pilot assemblies that has three small wires going to it with one large red spark wire? If so its not uncommon for the pilot orifice to get restricted or the bimetal to stick on the pilot assembly. The only way to find out is to remove the assembly and orifice to clean and blow it out. Put it back together and give it a try. You have to remove the pilot gas tube from the pilot assembly to get the orifice out to clean it.

Yes, it is a three wire pilot. I believe these answers have clarified for me how the whole thing works. My sensor and igniter are one piece.

Please note that this problem only occurs once or twice during the entire heating season. I never actually get to see the system not ignite and cannot force it to fail. So I cannot be sure if the pilot actually comes on during failure and it therefore is a sensor problem or whether the pilot is not igniting. But when I see the pilot ignited, it burns a nice blue, so I do not think it is clogged. I have tried cleaning the sensor/igniter in place but am a little afraid of trying to remove the assembly myself. The sensor/igniter is tough to get to so I am not sure how well I was able to clean it. I will just have to wait and see if it fails again.

Thanks for the help

Mikey42