OBD-II Heated Oxygen Sensor Numbering Convention

The FIRST oxygen sensor in cylinder bank #1 is numbered HO2S11.

The SECOND oxygen sensor in cylinder bank #1 is numbered HO2S12.

The FIRST oxygen sensor in cylinder bank #2 is numbered HO2S21.

The SECOND oxygen sensor in cylinder bank #2 is numbered HO2S22.

Note:
- Cylinder bank number one is the bank that contains the number one cylinder.

- If a V-6 or V-8 vehicle only uses one downstream Heated Oxygen Sensor (HO2S) it is numbered HO2S12, even if it uses two upstream HO2S.