Upstream Oxygen Sensor
Upstream oxygen sensor is a heated oxygen sensor mounted on the exhaust manifold. See Fig 1 and Fig 2 . 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). The heating element in the sensor provides heat to the sensor ceramic element. Heating the sensor allows the system to enter closed-loop operation sooner. It also allows the system to remain in closed-loop operation during periods of extended idle. In closed-loop, the PCM adjusts injector pulse width based on the upstream heated oxygen sensor input along with other inputs. In open-loop, the PCM adjusts injector pulse width based on preprogrammed (fixed) values and inputs from other sensors.