Dirt and moisture act to plug up the pressure taps in the motor housing. This will eventually result in the heater pressure differential switch shutting off the heaters gas solenoid valve-- as if the motor failed or the vent was blocked. This leads to a fault shutdown requiring the condition be cleared and pressing of the heaters manual reset button.
1) Pull red silicon hoses loose from the pressure differential switch, but leave connected to the motor housing.
2) Blow through each red silicon hose to clear the pressure differential (PD) taps. An eyedropper full of water (or a PD Tube Cleaning Bulb available through A & L Enterprises) squirted through the tube will loosen severe blockages. Then, again blow through the tube to clear the PD tap.
3) Reconnect the red silicon hoses to the Differential Pressure Switch.