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Reply to Bob Cook
Bob: Yes the heater OH light does come on. There's a new combustion air blower as well.
To my knowledge, the thermostat does not measure heater coil temperature, it measures airflow temperature in the cabin floor area under the pilot's toe (where the toe valve is). So the thermostat would cycle the heater based on airflow in the ductwork, but if the airflow is not adequate over the coil, then the thermostat would be calling for heat, the heater would be firing, but the coil would overheat.
Since the gremlin happens every time with the same pattern, i.e. runs for 20 seconds, light comes on, heat shuts off, I would doubt an intermittent short, because then run time would vary. On the other hand, it seems to run longer on the ground. sigh.
Maybe it is some vibration induced problem.
I'll let you all know what the inspection of the intake valve turns up, in the meantime I'll bring a blanket and hot drink on my next trip north. Thanks.
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