What is bank 1 in a Subaru Forester?

Answer 1

Subarus number their cylinders differently, perhaps because they're first and foremost a Right Hand drive car in their native Japan.

Bank #1 is the passenger side on an American Subaru, Bank #2 is driver side.

So if you throw an ECM code of P0051 (Oxy sensor heater problem - Bank#2, Sensor #1) you probably have to replace the Oxygen sensor on the driver's side of the car. Sensor 1 is the sensor BEFORE the catalytic converter. You'll have to get under the car to replace it, but it typically only involves unscrewing the old sensor, replacing it with the new, then connecting the wire for the new sensor in the same place as the old connection.