S&T Series Blazer, Bravada, Envoy, Jimmy, Rainier, Trailblazer, Xtreme

Heated Oxygen Sensor


The Heated Oxygen Sensor (HO2S) detects the presence of oxygen in the exhaust and produces a variable voltage according to the amount of oxygen detected. The HO2S outputs a voltage from 0-1v. A value less than 0.4v indicates lean A/F ratio and a value over 0.6v indicates a rich A/F ratio.


In the V6 and V8 examples in the Graphic above, HO2S-11 refers to the upstream oxygen sensor and HO2S-12 refers to the downstream oxygen sensor. The downstream oxygen sensor or third oxygen is referred to as HO2S-13. The upstream HO2S-11 (or HO2S-12 on the V6 and V8 engines as noted) signal is used with the Oxygen Sensor Monitor test function. The HO2S-12 (or HO2S-13 as noted) signal is used with the Catalyst Monitor test function.

HO2S-11, Hos-12, HO2S-13 Locations - Example 1

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).

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Fig. HO2S Location Graphic