Sta-Rite 400N won't stay running

[tyoungpsp]

My Sta-rite Maxitherm heater starts up fine, the blower is blowing good, and ignites shortly after starting. The unit will run for about 5 mins then click and cycle off the flame. Approximately 30 seconds later it clicks and ignites again. In under 30 seconds, it cycles off the heat again. Each time it cycles it takes less time for it to cycle off. After about 5 cycles the service heater light comes on with no error codes on the back of the board.

The ignition control board flashes once right as it clicks to shut off the flame. The chart says "Air Flow Fault"

The air flow switch did test out bad so I replaced it with a spare Pentair switch I had laying around. The spare tests good (measuring 0 resistance) with an ohm meter while attached to the tubes and the blower is running.

I have also replaced the Thermistor due to the temperature reading sporadically, the thermal regulator due to a rattling noise from the thermostat disconnecting from the spring (unsuccessful keeping it reattached), and the high limit switch since the old one was corroded. I have swapped the main board and flu sensor from a known working heater. I have bypassed the water pressure switch and the gas shut off switch for testing purposes

The heater resets when power is cycled off and back on. Always runs great at first then cycles off.

Any helpful hints would be appreciated.

 

[PTS]

With this one, i'd recommend checking the tubes for any cracks, or blockages. Make sure that the air orifice on the blower is intact and still has the 'flower'. Beyond that, check the exhaust to ensure no blockages exist there. Last would be to pull one of the manifold bolts off the header, you've got 5 on either side of the plastic header. If water comes out, there's a big problem. If not, the last thing to look at would likely be the ICM.