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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.
same way you replace any sensor.
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Inspect the wiring to the bank 2 knock sensor, if the wiring looks okay replace the bank 2 knock sensor.
driver side bank 1 before cat
Unplug then unscrew the old one. Screw the new one in and plug it into the wiring harness. Special sockets that will help you to remove the oxy sensor are available for loan at many auto parts stores.
Replace bank one O2 sensor
usually bank 1 is the bank with # 1 cylinder in it and sensor # 1 is the front o2 sensor(usually in the manifold). You will need a O2 sensor socket or a 22mm wrench or deep socket.
O2 sensor circuit bank 1 sensor 2 no activity. Check the wiring and connections to bank 1 sensor 2 O2 sensor, if all looks good replace the sensor.
Replace it. Sensor 2 is the one AFTER the catalytic converter.
Replace it.
It would be the cylinder head or valve cover that sits closes to the front of the engine