Mastertemp 400 cycling No codes or service light

The hsi alternates between igniting and flame sensing. I wouldn’t change anything else until you check gas wc. You’re looking for a static drop under 4” wc.
Solved!!! While waiting to get my hands on a manometer I pulled the igniter. It looked fine besides a little green discoloration on the end. I lightly brushed it cleaning it off and reinstalled it and the heater is back working and not cycling anymore. I should of taken pictures of it but honestly didn’t think it was going to be the issue when I was inspecting it.
 
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