Electronic Ignition Problem—Main Gas Shuts Off after 20 Minutes

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New Member
Feb 16, 2021
I have set up my gas fireplace with a Robertshaw 780-715U Ignition Control Unit and a Model 1751-749 Ignitor Sensor, sparking to a Model 1830-010 pilot light assembly. When energized, it starts up and runs fine for 20-30 minutes. Then the main gas turns off, while the pilot flame remains on and the sparking starts up again—but to no avail in turning the main gas back on.

By the time of the shut down, the wire probe in the Ignitor Sensor has become glowing red from being in the pilot flame (and also in the main burner flame). My suspicion is that it is becoming overheated and not conveying the rectified flame signal back to the controller, telling the controller that the pilot light is on and that it is okay to turn on the main burner. When I let things cool down for a while, everything works fine for another 20 minute cycle.

Any thoughts as to what the problem is and what I need to do to fix it? Is it normal for the Sensor to get red hot? Should it (and the whole pilot assembly) be located outside of the main burner flame?
The firebox is not enclosed and I think you would classify it as a b- vent perhaps. It is an old wood burning fireplace, with an 8” damper and vertical flue. It is used just for aesthetics, not heating.