What Is Oxygen (O2) Sensor?
An oxygen sensor measures the oxygen content of exhaust gas. The upstream sensor (before the catalytic converter) provides feedback for fuel mixture control; the downstream sensor monitors converter efficiency.
In plain English
The upstream O2 sensor is the engine computer's eyes for mixture: high oxygen means lean, low oxygen means rich. The computer uses this signal, second by second, to trim fuel delivery in closed loop.
Sensors age — contamination from oil, coolant, or sealants slows their response until the computer flags codes like P0133 (slow response). A sensor reporting inaccurately can also mask or mimic mixture problems, so its data is checked early in any fueling diagnosis.
Example
Related trouble codes
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