which is the bank 1 for the O2 sensor? on 2004 Hyundai Sonata

OBD codes 421

Asked by for the 2004 Hyundai Sonata
4cyl. Sonata? Front is B1 S1 , rear is B1 S2 .
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Bank one is on the head with number one cylinder.
On my car- there are two sensors and both are on the manifold. One is on the top of the manifold, the other is on the bottom of the manifold. Which is bank 1 sensor- since I only have one exhaust.