Today, we’re here to clear up some confusion between what Upstream vs Downstream O2 Sensors do.
In a vehicle’s exhaust system, the terms “upstream” and “downstream” refer to the location of the oxygen (O2)
An upstream O2 sensor is located before the catalytic converter, AKA on the upstream end of exhaust flow, and is used to measure the fuel / oxygen content in the exhaust before it passes through the converter. This information is used to adjust the air/fuel mixture in the cylinders on that bank to ensure that it is optimal for the engine, and the catalytic converter to function properly.
A downstream O2 sensor is located after or “downstream” of the catalytic converter and is used to monitor exhaust gasses after they have passed through the converter. The primary purpose of this sensor is to make sure that the catalytic converter is functioning correctly and that the emissions from the vehicle meet the required standards.
Upstream vs Downstream O2 Sensor: Summary
The upstream O2 sensor helps to optimize the air/fuel mixture for the engine based on the exhaust gas content before the converter, while the downstream O2 sensor confirms that the catalytic converter is functioning correctly and vehicle emissions meet expectations.
So, if your vehicle is presenting OBD2 codes related to the oxygen sensors, it’s important to get things squared away as soon as possible!
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