Upstream Oxygen Sensor

Upstream oxygen sensor is a heated oxygen sensor mounted on the exhaust manifold. Heater is used on upstream oxygen sensor so sensor reaches operating temperature quicker and maintains sensor at operating temperature during all operating modes. Heater on upstream oxygen sensor receives voltage from Auto Shutdown (ASD) relay. For operation of ASD relay, see RELAYS  under MISCELLANEOUS CONTROLS.

Upstream oxygen sensor produces a small electrical voltage input signal to Powertrain Control Module (PCM) depending on amount of oxygen content in the exhaust gas. Upstream oxygen sensor produces a low voltage when a large amount of oxygen exists in exhaust gas (lean condition) and a higher voltage when a lesser amount of oxygen exists in exhaust gas (rich condition). PCM uses input signal for maintaining proper air/fuel ratio by controlling fuel injector pulse width.