Bank 1 / Bank 2
Definition
In OBD-II terminology, Bank 1 is the side of the engine that contains cylinder number 1, and Bank 2 is the opposite side on V or flat engines; inline engines have only Bank 1. Sensor 1 is the lambda sensor upstream of the catalytic converter and Sensor 2 the one downstream, so B1S2 is the rear sensor on the cylinder-1 side; some engines with two catalysts per bank also have a Sensor 3. Many wrong parts are fitted because technicians assume Bank 1 is the driver's side or the front; always check the manufacturer's cylinder numbering, which varies even within one brand. On transverse V6 engines Bank 1 is frequently the rear bank against the firewall, which is also the harder one to reach. Codes identify the bank in their number: P0130-P0141 are B1 sensors, P0150-P0161 B2 sensors, P0171/P0172 B1 mixture, P0174/P0175 B2 mixture, P0420 B1 catalyst, P0430 B2 catalyst.