O2 Sensor

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📖 YOUCANIC Automotive Glossary

The O2 sensor measures exhaust oxygen content for ECU air-fuel ratio control. Upstream sensors (before the catalytic converter, Sensor 1) provide primary feedback for fuel delivery. Downstream sensors (after the converter, Sensor 2) monitor catalyst efficiency. A healthy upstream sensor switches between ~0.1V (lean) and ~0.9V (rich) as the ECU oscillates the mixture, while a healthy downstream sensor holds steady near 0.5-0.7V.

O2 sensor failures are among the most common check engine light causes. DTCs include P0130-P0167 (circuit codes) and P0420/P0430 (catalyst efficiency — often caused by a lazy downstream sensor, not a failed catalyst). Symptoms: decreased fuel economy, rough idle, rich (fuel smell) or lean (hesitation) running. Monitor live data to evaluate health — a healthy upstream sensor switches 6-8+ times per 10 seconds.

Synonyms:
Oxygen Sensor, Lambda Sensor
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