Combustion fan motor
The blower that supplies combustion air and pushes exhaust through the vent. When it slows, sticks, or stops reporting, the unit stops burning.
What it does
A condensing tankless heater is a sealed, fan-assisted appliance: the blower pulls in combustion air, pushes flue gas the full length of the vent run, and its speed is part of how the control board holds the air-to-fuel ratio. On Rinnai’s SENSEI and SENSEI RX units the venturi that mixes gas into that airflow mounts directly to the fan.
Why it fails
Bearings wear and the wheel slows. Debris — dust, insects, leaves drawn in through the intake — unbalances or jams the blower wheel. The check valve behind the fan housing sticks closed, which Rinnai lists as its own check on fan and airflow codes. And the winding opens, at which point the motor simply stops.
Not every fan-related code is a dead motor: a restricted vent produces similar symptoms, which is why blocked-air codes and fan motor codes share so much of their troubleshooting.
Why this is not a DIY part
Rinnai’s warning on this component is unusually blunt: never plug or unplug the motor while power is connected, or you risk damaging components including the motor itself. Reaching it means the front panel off, power and gas isolated, and the gas valve and venturi assembly disturbed — gas appliance work.
What a replacement usually costs
The motor is commonly $150-400. With labor, a fan motor replacement on a tankless unit typically lands between $250 and $900 depending on access and whether the board is implicated too.