DTC P0140: Ho2S2 Circuit - Insufficient Activity: Description

WARNING: This page is about a different car, the 2002 Saturn Vue. However, it is still accessible from the selected car via links, so may be relevant.

Heated Oxygen Sensor 2 (HO2S2) is an electrical source that responds to oxygen content out of the catalytic converter. When sensor reaches approximately 600°F (316°C), it produces a voltage based on the difference in oxygen between atmosphere and exhaust gas. ECM sends a bias voltage (425-460 millivolts) on signal line, which is pulled up through high resistance. When HO2S2 is cold, it produces no voltage and has extremely high resistance. Internal resistance of sensor is much more than resistance of bias pull-up resistor. However, when sensor heats up, it produces voltage that overrides bias voltage. This voltage is read by ECM to determine efficiency of catalytic converter. Under normal conditions, low sensor voltage means high oxygen content/lean air/fuel mixture and vice versa. Normal sensor readings at steady cruise with a properly functioning catalytic converter will be around 500-800 millivolts. DTC sets when HO2S2 signal is inactive.