Where is the 02 Oxygen Sensor before the cadalic converter on a 2006 Hyundai Santa Fe

oxygen sensor ( O2) are located after the cat converter
the one in the exhaust manifold is a heated oxygen sensor (HO2S) and is a totally different operation sensor

passenger side front oxygen sensor

THE CRANK SENSOR IS LOCATED ON THE ENGINE BLOCK NEAR TRANSMISION BUT CALL YOUR DEALER THIS TRUCK HAVE A RECALL FOR THE SENSOR

you have oxygen sensor located at the exhaust manifold and there is oxygen sensor located at the catalytic converter.

Heated Oxygen Sensor (H02S)

The sensors are located in the exhaust system. On some vehicles, one sensor is located up at the exhaust manifold(s) and the other sensor is located down at the catalytic converter. On other vehicles, both sensors are located down at the catalytic converter.

check engine coolant sensor

You will need a 02 sensor SOCKET

Remove it like you would a spark plug

Reinstall new o2 sensor with a little anti seize compound ON THE THREADS ONLY DO NOT GET ANYTHING ON THE SENSOR TIP OR YOU WILL RUIN IT.

Torque to specified value for your model

Reconnect o2 sensor wire

Done.

o2 sensors will be inbedded in the exhaust system....typically just after the manifold and just after the catalytic converter. if you can get under there you will see a robust wire running to the exhuast pipe connecting to a protruding component that measures the volume of polutants after ignition, before catalytic conversion, and judst after. Many vehicles have just one 02 sensor to measure the "upstream" (between the engine and the cat) and all will have one after.
to replace the sensor simply unscrew the old one, and put the new one in its place.
]
]Just make sure you get the right one from the parts store.....they'll have it in their database

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Heated Oxygen Sensor The sensors are located in the exhaust system. On some vehicles, one sensor is located up at the exhaust manifold(s) and the other sensor is located down at the catalytic converter. On other vehicles, both sensors are located down at the catalytic converter.


This is the upstream sensor




This is the down stream sensor

code p 0421 refers to warm up cat converter bank 1 --- efficiency below threshold ===causes ---cat converter---wiring ---HO2S (heated oxygen sensor )
there are not 4 oxygen sensor as you have been told but just 2 oxygen sensors located after each of the cat converters
The other 2 are heated oxygen sensor which are located in the exhaust manifold or exhaust pipe before the cat converters
heated oxygen sensors do a completely different job to the O2 sensors in that they report to the ECM, the combustion gas mixture so the the ECM can adjust the mixture for max performance
The oxygen sensors (O2) read what is left after the cat converters have done their job and report to the ECM so that it can fine tune the mix
IF you didn't replace the correct cat converter then that will make the code come back
If you just replaced the O2 sensor , well that doesn't even figure in the problem