HaywardH250FDN IF Code that won't stop!

[TerriblyFrustrated]

Systems powers on, pump and filter restore water flow. System ignites and catches flame...but 5-10 seconds later it shuts down. After three attempts, the system goes into the default safety lockout.
Backstory: I had this issue last season but it went away after I cleaned the flame sensor. I also had this issue at the beginning of this pool season, which coincided with water under the heater (which I mistakenly though at the time was from rain getting in the shed). After playing some more with the flame sensor, it worked and then ran all season...with no water under the heater. Then a few weeks ago the water under the heater returned and sure enough, IF code. This time I couldn't get rid of it. So I tested everything I could think of: cleaned the orifices for spider webs, checked the ignitor (working)...however the water was definitely a leak. The heat exchanger was very rusted all over and had a leak. I got most of it cleaned and then took it to a radiator shop in town that inexpensively got the leak fixed. While I was servicing everything, I also pulled the burners out...they were all quite rusty with a lot of the holes covered. I cleaned out as much of that as I could to open up the holes...seemed fine. Ran a meter on the wires for the flame sensor, continuity is there. So I've troubleshooted Dang near everything I can. The unit is 10 years old (I've had it for two). The casing (spongy?) that houses the heat exchanger and the burners is still very wet (aka not dried out) from the leak that was happening. It's now too Dang cold up here in Canada for it to dry out on its own. That wouldn't have anything to do with it stopping the flame and brining on the dreaded Ignition Fault code...right? Any help would be greatly appreciated because I don't want to throw out the unit simply because I'm ignorantly missing something. TIA

 

[Poolbreh]

I think @swamprat69 would know how to properly test flame sensor.

 

[swamprat69]

To start off, if the material that you noted as (spongy) is the insulation that surrounds the burner box and heat exchanger it may deteriorate/crumble after being soaked and may need to be replaced to operate safely. It should be throughly dry before firing up the heater. The flame is sensed in current (microamps or uA) in what is called DC but is really a highly dampened AC Sine wave. The AC Sine wave is dampened because of the large difference in the area of the flame sensor (small area) and the grounding area (large area). The flame sensor should be cleaned with steel wool or a small steel/diamond file because cleaning with sandpaper or emery cloth can leave residue that will impede conductance. On the other side (ground side) all surfaces from the burner back to case ground must be clean bare metal (as the flame signal is sensed through the flame). This includes the burners and burner slots which can be cleaned with a wire brush and any other metal to metal surfaces back to case ground which may have lost conductance due to dirt/corrosion/oxdation. This would include orifices ( if the burners are hung directly on the orifices ), burner brackets and any other metal to metal breaks that occur between the burners and the case ground. If the grounding surface is reduced through dirt/corrosion/oxidation you are reducing the difference in area between the flame sensor and ground and the current can no longer be read as DC (highly dampened AC). If you need to check the flame current you can use a meter that will read micro amps DC by removing the flame sensor from the circuit board and placing the meter in series from the flame sensor wire to the flame sensor post on the circuit board.