Narrowband O2 Sensor

The factory oxygen sensor that only reads accurately right at stoichiometric, used by the ECU for closed-loop fuel trims.

A narrowband oxygen sensor is the stock front O2 sensor on most vehicles. It switches sharply around the stoichiometric point (about 14.7:1 on gasoline) and its voltage swings between roughly 0.1V (lean) and 0.9V (rich), but it cannot tell you how rich or how lean once you leave that narrow window. The ECU uses it in closed loop to nudge short- and long-term fuel trims toward stoich for emissions and cruise economy. It matters for tuning because it drives the trims you read while dialing in part-throttle VE, but it is useless for power AFR under wide-open throttle, where the engine runs richer than stoich and the narrowband simply reads pegged rich. Never trust it for a power tune; use a wideband instead. In HP Tuners VCM Scanner you log narrowband voltage plus short/long-term fuel trims to validate closed-loop VE corrections and confirm the sensor is switching normally.