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.
Downstream oxygen sensor produces a small electrical voltage input signal to Powertrain Control Module (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.