Once considered leading-edge technology, today's oxygen sensor has become a commonplace service item on modern vehicles. Popularly introduced in the 1970s, the Lambda or, as it's more popularly known, the oxygen sensor, rapidly became standard equipment as it's a vital component of closed-loop or feedback fuel control technology. By using a voltage input from the oxygen sensor, a powertrain control module (PCM) can verify that its fuel control strategy is producing a chemically perfect 14.7:1 (by weight) air-to-fuel combustion ratio.

Before we venture into basic PCM operating strategies and diagnostic procedures, let's review how a simple, single-wire zirconia oxygen sensor works in conjunction with the PCM. A closed-loop fuel control system must contain three essential components: an information processor (the PCM), an actuator (the fuel injector) and an exhaust gas sensor (the oxygen sensor). During a cold startup, the PCM operates briefly in an open-loop or fixed fuel-control mode until the oxygen sensor reaches at least 600° F operating temperature. In the open-loop mode, the fuel injector pulse width is controlled by engine speed, load and temperature.

The fuel control system enters closed-loop operation when the oxygen sensor reaches 600° F operating temperature and begins generating a voltage signal to the PCM (See Photo 1). After entering closed-loop operation, the PCM begins switching the air/fuel (a/f) mix...