How do I test my hayward flame sensor? - Hayward H Series Pool & Spa Heater 400k Btu Propane Gas Milivolt Ignition H400p1

As JerryFin mentioned, there are cracks in the solder on the circuit board edge connector that cause the unit to stop heating randomly. All you need to do is pull the board and re-flow the solder.

HF
Flame present with gas valve "OFF" error
If flame is sensed with the gas valve off the control will go into lockout. The blower will run until error condition is corrected. When corrected, control will run blower for 5 s then automatically restart after 2 minutes.
the error might be caused by a faulty flame sensor. i have heard of this happening the other way (carbon blocks sensor causing no ignite)

Very simple...
one complete bracket will drop straight down and then pull it out.
two small fasteners hold it in place...
one screw and one sm. nut. Located on the very bottom of inside of heater cabinet, Left side, below the burner tray. Nothin to take apart. Easy peasy.

click the link to go to that page, where you will see a picture of the bracket with the igniter and flame sensor. You will see the stud where you will remove the nut, and you will be able to see the hole where the screw is supposed to be, both securing it to the burner tray.

Hayward Ignitor Assembly with Bracket Series Heaters HAXIGN1931

Yes it does. Flame rectification is carried back through the ignitor assembly ground. Be sure all the points of contact are rust free between the ignitor assembly and the chassis.

Sounds like it could be the flame sensor.

Sounds like this is a flame sensor issue. It is not detecting the flame.