I had the same very exact problem and got my hands on the service manual for the Rinnai 1001 series heater to solve it. As fas as the spark gap it should be 3mm but I went through a lot of tests of the components and found that Solenoid 1 coil was open. It should read about 1.3K and mine was completely open. Solenoid 1 is the one against the back cover. You'll need a small inspection mirror to see it but it is part number C23B-23. REMOVE POWER/UNPLUG FROM THE WALL FIRST then pull the plugs off all of them and check the resistance of the coils, they all should be about 1.2-1.3K. If you havent already found the answer I hope this helps.
Also check the pressure switch, bottom right side in the front. Its a disk, diaphram type device about 4 inches in diameter with a microswitch on it. That switch should close, zero to 1 volts across it, once the combustion fan starts up and builds a little pressure. If it doesnt it could be the switch or the intake or exhaust parts of the flue or filters are clogged.
If you need further help, I’m available over the phone at https://www.6ya.com/expert/john_32f4b76dad4781c4
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