Oxygen Sensor Monitoring: Notes

NOTE: Testing oxygen sensor should be performed using oscilloscope from PRESET MEASUREMENT list. If signal remains high (rich condition) check: fuel injectors, fuel pressure, ignition system, input sensors that influence air/fuel mixture, and engine mechanical. If signal remains low (lean condition) check: air/vacuum leak, fuel pressure, input sensor that influences air/fuel mixture, and engine mechanical. A mixture related fault code should be investigated first and does not always indicate a defective oxygen sensor. See Fig 1.

Fig 1: View Of Bosch Oxygen Sensor

Courtesy of BMW OF NORTH AMERICA, INC.

For oxygen sensor to operate correctly, sensor element must be heated. A non-operating heater will not allow sensor signal to reach its predefined maximum and minimum thresholds, resulting in delayed closed loop operation causing an impact on emission levels. As part of monitoring function for heater current and voltage, circuit is also checked for an open, short to ground and short to voltage, depending on values of current or voltage being monitored. See OXYGEN SENSOR MONITORING CONDITIONS table.

On Bosch systems, heater monitoring function measures both sensor heater current and the heater voltage in order to calculate sensor heater resistance and power. Oxygen sensor heater current is calculated via a voltage drop over a shunt resistor, internal to control module. If power of heater is not within a specified range, a fault will be set. Next time heater circuit is monitored and a fault is again present, CHECK ENGINE light will be illuminated. Heater function is monitored continuously while vehicle is in closed loop operation, as long as heater is activated by PCM. See OXYGEN SENSOR VOLTAGE OPERATING RANGE table.

On Siemens systems, if heater output is too low, signal amplitude of oxygen sensor will be reduced. If minimum low (rich) voltage and high (lean) voltage cannot be obtained within a predefined time, a fault will be set. Next time heater circuit is monitored and a fault is again present, CHECK ENGINE light will be illuminated. Heater function of pre-cat sensor is monitored continuously while vehicle is in closed loop operation, as long as heater is activated by PCM. See OXYGEN SENSOR VOLTAGE OPERATING RANGE table.

OXYGEN SENSOR MONITORING CONDITIONS

Condition	Specification
Closed Loop Operation	Yes
Engine Coolant Temperature	Operating Temperature
Vehicle Road Speed	3-50 MPH
Secondary Air Injection	Not Active
Catalyst Temperature	Greater Than 661°F (350°C)
Throttle Angle Deviation	Steady Throttle
Engine Speed Deviation	Steady/Stable Engine Speed
Average Lambda Value Deviation	Steady/Stable Load

OXYGEN SENSOR VOLTAGE OPERATING RANGE

Application	Engine	Operating Voltage Range
3-Series (E46) & X5
Siemens	6-Cylinder	0-.8
All Others
Siemens	6-Cylinder	.1-4.9