Circuit Description

PCM supplies a bias voltage (about .45 volt) on Heated Oxygen Sensor (HO2S) signal high and low circuits. Battery voltage is supplied to sensor heater when ignition is turned on. Sensor voltage changes from a bias voltage range to normal operation as sensor heater reaches operating temperature. Sensor voltage on a cold engine, typically, changes from a bias voltage to a voltage less than .3 volt as sensor reaches operating temperature. Sensor voltage could possibly increase to greater than .45 volt depending upon exhaust gas content.

PCM will run heater test only on a cold start (dependent upon coolant and intake air temperatures) and only once per ignition cycle. PCM will monitor sensor voltage when engine is started. PCM will determine how much time elapsed when sensor voltage increases above or decreases below bias range threshold.

Code P0161 will set if PCM detects too much time elapsed for sensor to enter normal operating range. Time required for sensor to reach operating temperature is based off of engine coolant temperature. If engine coolant temperature is -40°F (-40°C), sensor should reach operating temperature within 240 seconds. If engine coolant temperature is 154°F (68°C), sensor should reach operating temperature within 100 seconds.

Code P0161 will set when the following conditions are present.

• Engine is started.
• Intake air and engine coolant temperatures are less than 122°F (50°C).
• Intake air and engine coolant temperatures are within 11°F (6°C) of each other at start-up.
• TP sensor angle is less than 94 percent.
• Sensor voltage remains at .3-.7 volt for more than 240 seconds if engine coolant temperature is -40°F (-40°C), or 100 seconds when engine coolant temperature is greater than .154°F (68°C).