I have an AO Smith FPS-75 series 230. This is a gas-fired 75 gal heater with spark ignition and induced draft, about 15 years old.
The problem is that it occasionally (once every few days)fails to ignite. If I reset the power, it starts up and works normally for a few days.
I can't troubleshoot it because once I turn it off and back on everything is normal. If I understand it correctly, it tries to ignite 3 times before it locks out permanently.
I had the same problem last year and found the spark gap to be too large. Closing it solved the problem. This time the spark gap is fine.