DTC P0140: Ho2S2 Circuit - Insufficient Activity: Description

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 difference in oxygen between atmosphere and exhaust gas. PCM sends a bias voltage (426-461 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 greater than resistance of bias pull-up resistor. When sensor heats up, it produces voltage that overrides bias voltage. This voltage is read by PCM 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, not fluctuating out of 426-461 millivolt range.