Q&A: Q:What is Differences Between upstream and downstream O2 Sensor? A:The upstream oxygen sensor is located before the catalytic converter. This measures the level of pollutants coming directly from the engine. It also senses any raw, unburned fuel coming from the combustion chambers. The downstream oxygen sensor is located directly after the catalytic converter. This sensor measures the pollutants making it through the converter and out of the tailpipe. The data from this sensor is compared to that of the upstream sensor. If the data between the two sensors becomes more similar, the vehicle's computer may trigger a catalyst inefficiency check engine code, which turns on the Check Engine light. .
