Furnace won't run properly...

[AeroMoto]

Lennox Conservator III G16 Upflow model
Robert Shaw Controller SP735L
Robert Shaw Grayson 7100 DER-S7C regulator
I believe the fan and limit controller is a Honeywell of some kind.

The inducer motor turns on and runs just fine. I can hear the ignitor click on. A few seconds later the burners kick on and then shut off after a few seconds. It seems to me that the flame sensor (or thermocouple?) is not functioning properly. The main blower never kicks on I'm assuming because the flame sensor is malfunctioning and not detecting a flame. So how do I troubleshoot it? I'm not quite sure where it's located, but any help is appreciated. It's cold in the house!

 

[domain]

Usually near the last of the burner tunes. This insures that they all light.

 

[Rockhead261]

Clean flame sensor with a Scotchbrite pad. Done.

 

[AeroMoto]

I cleaned the flame sensor and ignitor with the scotch brite pad, and the same results. The flame sensor had cracked ceramic insulation and I could move the metal probe up and down about 1/8". So I'm thinking the sensor is cracked and bad.

 

[zmaxmotorsports]

Do not clean hot surface igniter,sounds like flame sensor needs to be replaced if its broke loose like that.

 

[AeroMoto]

It's not a hot surface ignitor. It's a spark plug like one.

 

[TOOL FANATIK]

the problem does sound like the flame detector is not detecting a flame; we can all agree on that. now what is the root cause? it could be the actual flame detector is bad. im assuming this is a gas furnace?