Ignitor not quite igniting

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I have a 13 or so year old Rheem combined heat/cooling system on the roof and we tried the heating on the first cold day a couple of days ago, and no heat. Ignition is by Intermittent Pilot. The controller is a Honeywell 8600. When you turn the heat on, the spark sparks, the pilot light lights immediately then quickly goes out again. This goes on a few cycles, then the main gas appears to come on anyway. You can hear it and smell it. It doesn't light because the pilot never stayed lit long enough. The controller has a position for a "sense" connection, but no actual terminal there. I can't see anything that's giving the controller feedback that the pilot is lit, thus allowing the controller to open the main valve, which is apparently how these controllers are meant to work. But, it's worked fine in the past. Any suggestions as to what to replace, or try next?

Thanks

OP