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Do you get any other error code when it shuts down prematurely?
OK usually if heater is turning off before thermostat is satisfied, a limit or pressure switch is opening but we should get a different code if it does.
I would start by cleaning flame senser . If that doesn't solve, try to catch it fail and immediately ohm the limits and pressure switch to see which one is open if any
Do you have a meter to see if transformer is bleeding down?
Transformer should have 4 wires.2 supplying high voltage to transformer and two (going to relays and thermostat) 24 volts. If thermostat is issue, the 24v will drop.
Nope.......LOL. Give me a minute to pull the manual and see if it shows me.
Looks like red and brown going to SEC and COM terminals on board
Let me know what you find
Thanks for letting me work with you to this point