Robertshaw 780-715 opens main valve first time, but additional times only sparks

I think the title may be a bit misleading but wasn’t sure how to shorten it up. Here’s what’s happening:

I have a robertshaw 780-715U ignition control unit on a gas furnace. When heat is called for the first time (usually in the morning), the igniter sparks, the pilot lights, 5 seconds later, the main valve opens and all is good. The house warms up and the call for heat turns off. The problem then starts with the second cycle. The second call for heat has the ignitor spark and the pilot light but no main valve opening. After the pilot lights, the spark stops, but then resumes about 5 seconds later. This unit is configured with a combined sensor/ignitor.

I’ve tried replacing the sensor ignitor but the symptom is the same (purchased robertshaw 1751-729). I also tried using A white rodgers 50d50-843 but that doesn’t spark at all though the pilot valve opens on that unit.

In trying to diagnose the 780-715U, I check voltage between the main valve lead and the "white MV/PV" lead and I do not get any DC voltage when I would expect the main valve to be open. I have to say that I’m not 100% sure about that voltage reading because the ignitor is sparking and I’m a little nervous about getting close to that ignitor lead.

While I’m okay with buying another 780-715U, I had assumed the white-rodgers would be okay.

Do you all have any suggestions for further troubleshooting?


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