Where is the O2 sensor for code PO133 on a 4.2L V6 Chevy Trailblazer? on 2003 Chevrolet Trailblazer

How many O2 sensors are on this vehicle?

On your 2003 Chevrolet Trailblazer code p0133 is referring to the "front" oxygen sensor which is located in the exhaust manifold. Your vehicle uses only two oxygen sensors, the one in the exhaust manifold and another it the exhaust system, after the catalytic convertor.
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Hello the answer to your first question of the location for the oxygen sensor for fault code p0133 is in the exhaust manifold on the right side of the engine which is the same side for the number one cylinder.The answer to your next question is there is four heated oxygen sensors on your vehicle and all of them are within the exhaust system.I would make sure you have a professional tech.performed a complete diagnostic to confirmed this is the actual problem becuase vac./exhaust leaks and other failures can cause this fault code.You should also check your mileage due to the emission warranty is 8yrs./80,000 miles.HAVE A BLESS DAY from DR.RECO