What does the downstream O2 sensor do? Here's the answer - Jeep Cherokee Forum

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What does the downstream O2 sensor do? Here's the answer

Old 05-18-2018, 07:35 PM
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Default What does the downstream O2 sensor do? Here's the answer

This has been debated many times. A couple of years ago someone posted this in a thread, and I saw it then, but couldn't find it again to cite it. Since this gets people confused and arguing, I thought I'd post this here so people can more easily refer to it. This is from the 97 FSM, and should apply to all 97 and up (except California emissions). I suspect it probably applies to earlier models too. California vehicles with your mess of 4 (or is it 27?) O2 sensors may not work the same. You guys are just in a world of hurt.

Short story - the downstream 02 sensor does not affect the operation of the engine. It's just there to assess the condition of the cat.

14 - 30 FUEL SYSTEM
DOWNSTREAM HEATED OXYGEN SENSOR
The downstream heated oxygen sensor is located near the outlet end of the catalytic convertor (Fig. 6). The downstream heated oxygen sensor input is used to detect catalytic convertor deterioration. As the convertor deteriorates, the input from the downstream sensor begins to match the upstream sensor input except for a slight time delay. By comparing the downstream heated oxygen sensor input to the input from the upstream sensor, the PCM calculates catalytic convertor efficiency. When the catalytic converter efficiency drops below emission standards, the PCM stores a diagnostic trouble code and illuminates the Malfunction Indicator Lamp (MIL). For more information, refer to Group 25, Emission Control Systems.
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Old 05-19-2018, 07:20 AM
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Yep. Thats it. Computers were introduced into automobiles as a requirement by the EPA. The transition from OBD 1 to OBD 2 was mandated by the EPA. The major OBD 2 requirements were more monitoring of emission systems by the computer and a common plug, software, and code list.
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