📖 What It Does
Tells the ECU whether the engine is running rich (too much fuel) or lean (too much air). Without an O2 sensor, the ECU runs default open-loop fueling that wastes gas and pollutes more.
⚙ How It Works
A zirconia ceramic element inside the sensor produces a small voltage (0–1V) based on the oxygen difference between exhaust gas and ambient air. The ECU reads that voltage hundreds of times per second and trims injector pulse width to keep the mixture near stoichiometric (14.7:1). Wide-band sensors give a continuous reading instead of switching.
⚠ Symptoms When It Fails
Symptoms include the check engine light, poor fuel economy, hesitation, P0130-P0167 codes, P0420, or a failed emissions test. Sensors typically last 60,000–100,000 miles and are slowly poisoned by oil, coolant, or silicone contamination. See full symptom guide.
💰 Replacement Cost
Replacement: $150–$500. A sensor is $50–$250 in parts. Labor adds $50–$200, depending on how easy it is to reach. See O2 sensor cost.