What do they call an oxygen sensor on the front and back of a catalytic converter?

Front of the converter is upstream back of it is downstream. Upstreams are generally referred to as sensor 1, downstreams as sensor 2. Bank 2 sensor 1 would refer to the upstream sensor on the exhaust from the left side of a V engine. V engines sometimes have more than one converter and up to 4 oxygen sensors.

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It all depends on how many sensors it has. If it is a v8 the driver side sensor before the cat is the bank 1 sensor 1 and the passenger side one is bank 1 sensor 2. The one behind the cat is bank 2 sensor 1. Some newer cars have one in the cat and i think that is bank 3 sensor 1, but i maybe mistaken.

Is your sante fe 4 or 6 cyclinders? But looking at the PO051 code it means it is the one after the cat.

bank 1 sensor 1 will be on the same bank as cylinder 1 and it is the front 02 sensor, bank 1 sensor 2 is still the same side but the 02 sensor behind the cat, bank 2 sensor 1 is the opposite side front 02 sensor and bank 2 sensor 2 is the one behind the cat on that side, bank refers to which side of engine it is on and sensor 1 is front and sensor 2 is behind the cat.

Almost got all of the information! Need to know the engine option of your Santa fe: the 2.4L 4 cylinder or the 2.7L DOHC V-6? ADDED; Thanks for the information! The sensor that you need to replace is the front bank and the one closest to the engine!