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Heated Oxygen Sensor (HO2S)

NOTE: Models are equipped with either 2 or 4 HO2S. Some federal emission vehicles will have 4 HO2S. See wiring diagram in WIRING DIAGRAMS article for vehicle information on upstream and/or downstream HO2S.

HO2S produces a small electrical voltage (0-1 volt) when exposed to heated exhaust gas. HO2S is electrically heated for faster warm-up. Heating element is powered through Auto Shutdown (ASD) relay.

HO2S acts like a rich/lean (air/fuel ratio) switch by monitoring oxygen content in exhaust gas. This information is used by PCM to adjust air/fuel ratio by adjusting injector pulse width.

HO2S produces low voltage when oxygen content in exhaust gas is high. When oxygen content in exhaust gas is low, HO2S produces a higher voltage. OBD II system uses a second HO2S after converter to monitor converter efficiency.

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