Downstream Oxygen Sensor
Downstream oxygen sensor is a heated oxygen sensor mounted on exhaust pipe behind catalytic converter. See Fig 1 . Heater is used on downstream oxygen sensor so sensor reaches operating temperature quicker and maintains sensor at operating temperature during all operating modes. Heater on downstream oxygen sensor receives voltage from Auto Shutdown (ASD) relay. For operation of ASD relay, see RELAYS under MISCELLANEOUS CONTROLS.
The downstream heated oxygen sensor input is used to detect catalytic converter deterioration. As the converter deteriorates, the input from the downstream sensor begins to match the upstream sensor input except for a slight time delay. By comparing the downstream heated oxygen sensor input to the input from the upstream sensor, the Powertrain Control Module (PCM) calculates catalytic converter efficiency. Also used to establish the upstream O2 goal voltage (switching point).
Downstream oxygen sensor produces a small electrical voltage input signal to PCM depending on amount of oxygen content in the exhaust gas. Downstream 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 to monitor catalytic converter. As catalytic converter deteriorates, input signal from downstream oxygen sensor begins to match input signal from upstream oxygen sensor except for a slight time delay. PCM compares input signals from upstream and downstream oxygen sensors to calculate efficiency of catalytic converter. If efficiency of catalytic converter becomes less than specified value, a diagnostic trouble code will be stored in PCM.