Part 7

7 EXHAUST GAS SENSOR MONITORING Contd.

7.11 Heating of the Secondary Oxygen Sensor

7.11.1 General Description
The purpose of the oxygen sensor heater is to heat the oxygen sensor to its operating temperature as quickly as possible, and then, depending on the environmental conditions, to have the sensor operated at a ceramic temperature as constant as possible, in order to attain a high accuracy of the lambda control.

7.11.2 Rationality Check: Internal Resistance Too High - P0141/P0161
P0141 - Bank 1, Sensor 2
P0161 - Bank 2, Sensor 2
P3218 - Bank 3, Sensor 2
P3248 - Bank 4, Sensor 2
P0147 - Bank 1, Sensor 3

Monitoring Strategy
The internal resistance of the Nernst cell is compared to a setpoint value.

Typical Enable Conditions
- Engine-off time > threshold value
- Intake air temperature > threshold value
- Modeled exhaust gas temperature within a defined range
- Deceleration fuel cut-off not active
- Heater control active

Malfunction Criteria
The heating performance monitor requires a sufficiently filtered steady-state value of the internal resistance. For this, a defined number of individual measurements must be reached after the operational readiness of the sensor, and not more than 8 seconds may have passed since the last individual measurement.

If the sensor heater is defective, the internal resistance is considerably greater than with a properly functioning heater. If the internal resistance of the Nernst cell exceeds a setpoint value that depends on the engine operating point (the exhaust gas temperature and the exhaust gas mass flow) and the operating conditions of the sensor, the sensor is detected as defective.

Determination of the Loaded Sensor Voltage, the Unloaded Sensor Voltage, and the Internal Resistance
By using a pulsed pump current the internal resistance of the oxygen sensor's Nernst cell can be determined which may also be used as a criterion for the electrical wiring diagnostic.






The load resistance RB is dimensioned in a way that a pump current of 0.5 mA flows into the oxygen sensor. Measuring the internal resistance is effected as follows:






In order to determine the internal resistance Ri of the Nernst cell, the unloaded and the loaded sensor voltages are measured. The last measured voltage value before the activation of the voltage pulse is stored. Applying the current pulse to the oxygen sensor causes a voltage drop at the internal resistance of the Nernst cell, and thus an increase in the sensor voltage.






7.12 Heater Output Stage Monitor of the Secondary Oxygen Sensor

7.12.1 General Description
The purpose of the oxygen sensor heater is to heat the oxygen sensor to its operating temperature as quickly as possible, and then, depending on the environmental conditions, to have the sensor operated at a ceramic temperature as constant as possible, in order to attain a high accuracy of the lambda control.

7.12.2 Short to Battery / Short to Ground / Open Circuit P0038/P0058 P0037/P0057 P0036/P0056

Short to Battery: P0038 - Bank 1, Sensor 2 P0044 - Bank 1, Sensor 3
P0058 - Bank 2, Sensor 2 P3216 - Bank 3, Sensor 2 P3246 - Bank 4, Sensor 2
Short to Ground: P0037 - Bank 1, Sensor 2 P0043 - Bank 1, Sensor 3
P0057 - Bank 2, Sensor 2 P3215 - Bank 3, Sensor 2 P3245 - Bank 4, Sensor 2
Open circuit: P0036 - Bank 1, Sensor 2 P0042 - Bank 1, Sensor 3
P0056 - Bank 2, Sensor 2

Monitoring Strategy
The heater voltage is compared to an upper and lower threshold value.

Typical Enable Conditions
- Engine speed > threshold value
- Heater control active

Malfunction Criteria
If the heater voltage exceeds a threshold value, a short circuit to battery is detected.
If the heater voltage falls below a lower threshold value, a short circuit to ground is detected.
If the heater voltage is greater than a lower threshold value AND less than an upper threshold value, an open circuit is detected.