There are 3 O2 sensors on the '03' Chevy Blazer. 2 upstream of the CAT and 1 downstream of the CAT (Which is the easiest to find). The two upstream are the "bank 1" (driver side) and "bank 2" (passenger side) Sensors. They are a little more difficult to locate but if you trace the exhaust manifolds off each side of the engine, you will see them tucked up in there between the wheel well.
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