Recently I had an issue with my Hayward H400ND1 Induced Draft Gas Pool Heater (don’t say that three times fast). The heater no longer would kick on (or even try to turn on) even though the “System Normal” light stayed lit. I couldn’t find much information online about my particular model and type of heater. After testing the normal components (limit switches, pressure switches, thermostats, on/off switch, transformer, gas valve, etc.) I ended up re-crimping some connectors on the wires and replacing a 22MΩ resistor (on the igniter control board) that was reading at 26MΩ.
To see some details, check out my YouTube video. Hope it is helpful to you.