We're having an issue with our Hayward H400. Pool was closed for 3 seasons, just finally opened it again. Heater is dual thermostat (pool/spa), yellow "System Ready" and green "Heating" light, twin-position switch (pool/off/spa), 8 combustion chambers, with DSI. No water leaks, a little dusty, many webs. Gas pressure is good, valve functions normally. Would not light at first, verified DSI spark generator is working. Now it'll light about every 5th time, with a big poof. When it doesn't light the gas valve will close after about 2-3 seconds, if it does light the gas valve will close after about 5 seconds after lighting.
Guessing either the gas jets, or the combustion chambers, creating a bad burn environment. Do either of these go "bad" (corrode, chip, open/close holes, etc), or are they manufactured enough that a good cleaning will fix these issues? The gas nozzles appear removable without much work, it appears to repair/replace the chambers will require removing the gas valve/piping (ugh!).
Any ideas on why the unit is turning off after 5 secs? Reviewing the wiring diagram (inside of upper control panel), it doesn't appear to show any flame sensors. There appears to be two thermo sensors, in-line with two water pressure sensors, in-line with a water temp sensor, leading to the "System Ready" yellow light. Yet the diagram shows "closed" being normal for the thermo sensors, and "open" for the water pressure sensors, so I'm presuming they are over-heat sensors, not flame-sensors. Where's the flame sensor?