Front, or upstream 02 oxygen sensor(s):
The right and left upstream oxygen sensors are typically labeled Bank 1, Sensor 1 and Bank 2, Sensor 1 located in the exhaust manifold or in the downpipe before the catalytic converter.
more info.
Rear, or downstream 02 oxygen sensor(s):
The right and left downstream oxygen sensor(s) are typically labeled Bank 1, Sensor 2 and Bank 2, Sensor 2 and located in the pipes after the catalytic converter(s).
note: some vehicles will not have 2 cats and 4 sensors like my Silverado.
