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In the exhaust system, before the muffler.
There is one in front of and one behind the catalytic converter.
It has oxygen sensors before and after the catalytic converter.
In my '96 K2500 it is between the exhaust manifold and the catalytic converter.
take the bolts off. drop the old install the new one.
That would be the O2 sensor after the catalytic converter.
O2 sensors are usually on the exhaust manifold. Actually on the '96 Galant, one is in the header pipe (the pipe running below the engine between the manifold and the catalytic converter) and the other is in the catalytic converter.
Plugged catalytic converter
There are two of them, one on either side of the catalytic converter in the exhaust system.
After googling it is determined Code P0420 ( Catalytic converter effeciency below threshold)
Clogged converter, running too rich or a misfiring plug.