In both of the pictures in Graphic above, HO2S-11 refers to the upstream oxygen sensor while HO2S-12 refers to the downstream oxygen sensor.
In these examples, the upstream HO2S-11 signal is used with the Oxygen Sensor Monitor test function. The downstream HO2S-12 signal is used with the Catalyst Monitor test function. HO2S location information is very important when attempting to identify the correct oxygen sensor as it relates to a trouble code repair chart.
HO2S-11, Hos2 Locations - Example 2
Throughout the manual, there are references to Heated Oxygen Sensors identified with acronyms HO2S-11, HO2S-12 and HO2S-13. In addition, the sensors are identified as either cylinder Bank 1 or cylinder Bank 2. It should be understood that Bank 1 always contains engine cylinder number 1 (Cyl 1).