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I repair my "Reddy Heater" torpedo heater by installing a new photocell/flame sensor. It's a very simple procedure.
Original test video:

All you really have to do is take off the top shielding, the side cover, and then remove the photocell by pulling it off, or cutting it. Then remove the photocell connections, pull it out and replace.

Photocells on eBay: #PP236:

Owners manual:

Reddy Heater Torpedo Kerosene Heater. 50,000 BTU Model: R50A