Oxygen sensors are numbered by which bank of cylinders they serve. Bank 1 is the left or drivers side bank of cylinders on the 4.3L which has the odd numbered cylinders 1,3, and 5. Bank 2 is the right or passenger side bank of cylinders numbered 2, 4, and 6. The Bank 1 Sensor 1 would be the sensor for the left hand side of the engine AHEAD of the converter. Bank 1 Sensor 2 is the sensor for the left hand side bank of cylinders BEHIND the converter.
Fig. Fig. 3: Oxygen sensor-4.3L engines
Fig. Fig. 4: Oxygen sensor-5.0L and 5.7L engines
Fig. Fig. 5: Oxygen sensor-7.4L engines
Oxygen sensors are always numbered like this:
Bank 1 sensor 1
Bank 2 sensor 1
Bank 1 sensor 2
Bank 2 sensor 2
Some manufacturers use a kind of shorthand that reads different, but means the same thing:
Sensor 1/1 or O2s 1/1
Sensor 2/1 or O2s 2/1
Sensor 1/2 or O2s 1/2
Sensor 2/2 or O2s 2/2
Bank 1 is always the side of the engine where cylinder number 1 is located and, of coarse, Bank 2 is the opposite side.
On a 4 cylinder engine, there is only 1 bank and it is always referred to as Bank 1.
Sensor 1 is always the upstream sensor (the one located BEFORE the catalytic converter)
Sensor 2 is always the downstream sensor (the one that is located AFTER the catalytic converter.
Hope helps (remember to rate this).
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