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My home while I was working in the dining room heard two bangs. I looked around and on my second trip to the basement , I saw that my PH100-55 had a F09 fault. So I cleaned the flame rod. No go! I took out the ignitor assembly. Corroded by humidity in the combustion chamber. I got a new ignitor assembly. No go for spark! Removed it and checked the gap. The gap is perfect. No spark as the unit is in lockout. I spent 30 minutes finding the way to reset in the manual. Bingo, reset worked after pressing and holding 4 NOT as per manual of only pressing 4. All is good!