Diagnostic Trouble Codes (DTC)
When oxygen sensor failure occurs, a DTC is recorded in the Engine Control Unit (ECU) and a Malfunction Indicator Lamp (MIL) is illuminated on the dash, alerting the driver the vehicle has a problem.
To diagnose the fault, a code reader or scan tool is connected to the vehicle to read the trouble code. The scan tools can vary in the display of information shown. Some show a definition for the code while others show only a trouble code number. There are generic or standard OBD II codes and vehicle manufacturers use additional codes called enhanced or OEM-specific codes. On many older (pre 1995) vehicles, a trouble code or DTC can be read without a scan tool or code reader by using a manual flash code procedure.
After having identified the description using a list of DTC trouble codes, the next step is to diagnose the fault. You must follow the diagnostic procedure to properly diagnose the system, sensor and/or circuit.
The trouble code itself does not tell you which part to replace! The scan tools or code readers speak OBD II language, meaning the references to the engine are coded.
B1 = Bank 1
B2 = Bank 2
When applying OBD II to oxygen sensors, the codes would then read B1S1. This refers to Bank 1 Sensor 1.
Finding Bank 1 is not difficult. The front of the engine will have the accessory pulleys and drive belts, regardless of orientation in the engine compartment. Bank 1, containing cylinder #7, is always the most forward cylinder on the block. There will be a visible difference in the cylinder head location.
Sensor 1 will be the pre catalytic position and Sensor 2 would typically be the post catalytic position. In some instances, Sensor 2 can be pre catalytic thus making Sensor 3 post catalytic.