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Part 4

7 EXHAUST GAS SENSOR MONITORING Contd.

7.5 Monitoring of the Front Oxygen Sensor Heater

7.5.1 General Description
To provide a signal that can be evaluated, a wide-range oxygen sensor requires an operating temperature between 650 and 950 °C. The exhaust gas is not capable of increasing the oxygen sensor temperature to this temperature level quickly enough, particularly during the engine warm-up cycle. For proper function of the Oxygen sensors, their ceramic elements must be heated. A non-functioning heater delays or prevents the sensor readiness (LSU) for closed loop control and thus influences emissions.
For this reason, an electric heater with a closed-loop control is used to increase and, later on, to maintain the operating temperature in the sensor ceramic. Depending on the control difference, the heating output is increased or decreased via pulse-width modulation (PWM) by adapting the pulse duty factor (ratio of the on-time and the period) of the PWM signal.






7.5.2 Heating Output Check, P0135, P0155, P3148, P3150
Malfunction: Temperature of the sensor ceramic is too low despite of maximum heater output

Master: Bank 1: P0135, Bank 2: P0155
Slave: Bank 1: P3148, Bank 2: P3150

Monitoring Strategy
While the heater control is active, the heating output AND the measured temperature in the sensor ceramic (actual value) are compared to defined threshold values.

Typical Enable Conditions
- Modelled exhaust gas temperature at the oxygen sensor > threshold value
- Heater control active
- No reduced heating output (to protect the sensor when exhaust gas temperatures are very high) requested
- Valid exhaust gas temperature model
- No fault present at the sensors that provide the input signals (e. g. ambient air temperature, vehicle speed) for the model.
- Monitor delay time after the oxygen sensor heater switching-on or after a deceleration fuel cut-off has elapsed

Malfunction Criteria
If the setpoint value of the ceramic temperature is not reached while the heater control is active, the output of the sensor heater is increased. If the heating output is greater than a defined threshold value while the heater control is active AND, while at the same time, the ceramic temperature is less than a defined threshold value, the fault "Temperature of the Sensor Ceramic too low despite Maximum Heating Output" is detected. If the internal resistance measurement is valid, the corresponding fault is stored in the fault memory. The internal resistance measurement is valid if the internal resistance measurement function is active and if no fault in the measurement circuit of the IC, no IC circuit fault and no LSU circuit fault are currently present. Additionally, for the oxygen sensor type LSU ADV the number of internal resistance measurements blanked out due to a requested increased pump current, must not be greater than a threshold value (typically 30).

7.5.3 Sensor Ceramic Heat-Up insufficient, P0135, P0155, P3152, P3153
Malfunction: Sensor ceramic heat-up is insufficient, so the operational readiness of the sensor and therefore activation of the lambda control is achieved too late

Master: Bank 1: P0135, Bank 2: P0155
Slave: Bank 1: P3152, Bank 2: P3153

Monitoring Strategy
To monitor the ceramic temperature of the Nernst cell after the engine start, the temperature measured in the sensor ceramic is checked after a defined on-time of the sensor heater.

Typical Enable Conditions
- Engine coolant temperature at engine start > defined threshold value
- Engine-off time during the preceding ECM keep alive time > defined threshold value
- Oxygen sensor heater active
- Temperature of the exhaust pipe near the oxygen sensor < defined threshold value
- Heater control active

Malfunction Criteria
When the enable conditions are fulfilled, a time counter for a monitor delay time is started, after which the sensor ceramic temperature is evaluated. This ensures that the operational readiness of the oxygen sensor is reached due to a sufficient heating output. This time counter is interrupted if not all of the injection valves are activated or if the vehicle system voltage is too low or too high for the operation of the sensor heater. When the time counter for the monitor delay time is interrupted, a time counter for the interruption time is started. If this time counter exceeds a threshold value, monitoring is not possible in the current driving cycle.

When the operational readiness of the oxygen sensor is reached, i. e. when the time counter for the monitor delay time has exceeded a threshold value, the temperature of the sensor ceramic is evaluated. If the ceramic temperature remains below a threshold value for at least a defined time period, the fault "Sensor Ceramic Heat-up insufficient, Operational Readiness of the Sensor and therefore Activation of the Lambda Control achieved too late" is detected.

If the measurement of the internal resistance is valid, the corresponding fault is stored in the fault memory. The internal resistance measurement is valid if the internal resistance measurement function is active and if no fault in the measurement circuit of the IC, no IC fault and no LSU circuit fault are currently present. Additionally, for the oxygen sensor type LSU ADV the number of internal resistance measurements blanked out due to a requested increased pump current, must not be greater than a threshold value (typically 30).

7.6 Circuit Monitoring Wide Range Oxygen Sensor (LSU) Heater Power Stage

7.6.1 Short to Battery/Ground or Open Circuit

Monitoring Strategy
For short to ground and open circuit:
The voltage at the output of the wide range oxygen sensor heater power stage is continuously monitored by the ECM.
For short to battery:
The current at the output of the wide range oxygen sensor heater power stage is continuously monitored by the ECM.

Typical Enable Conditions
For all monitors:
- Time since engine start greater than defined threshold value

For short to ground and open circuit:
- Control signal for wide range oxygen sensor heater "Off" is present

For short to battery:
- Control signal for wide range oxygen sensor heater "On" is present

Malfunction Criteria
Short to ground, P0031:
If the voltage at the output of the wide range oxygen sensor heater power stage in the ECM is in a defined lower range for a defined time period, a short to ground is detected and a fault is stored in the fault memory.

Short to battery, P0032:
If the current at the output of the wide range oxygen sensor heater power stage in the ECM is greater than a defined threshold value for a defined time period, a short to battery is detected and a fault is stored in the fault memory.

Open circuit, P0030:
If the voltage at the output of the wide range oxygen sensor heater power stage in the ECM is in a defined upper range for a defined time period, an open circuit is detected and a fault is stored in the fault memory.

7.7 Front Oxygen Sensor, Ceramic Temperature Monitoring

7.7.1 General Description
Enabling the diffusion of oxygen ions through the ceramic membrane in the Nernst cell of a wide range oxygen sensor (LSU) requires a certain temperature. Therefore, the Nernst cell is electrically heated when exhaust gas temperatures are too low. This monitoring function checks if the temperature of the ceramic membrane in the Nernst cell is sufficiently high.

7.7.2 Ceramic Temperature too low - Front Oxygen Sensor Circuit Fault, P0130

Monitoring Strategy
The temperature of the Nernst cell ceramic in the oxygen sensor is monitored.

Typical Enable Conditions
- Modelled exhaust gas temperature > defined threshold value
- Deceleration fuel cut-off not active

Malfunction Criteria
A circuit fault or heater fault cannot always be detected immediately by the corresponding monitor function. For this reason, also the ceramic temperature of the Nernst cell is monitored in order to detect a fault at the front oxygen sensor as quickly as possible. If the ceramic temperature of the Nernst cell falls below a defined threshold value, a circuit fault at the front oxygen sensor is detected and stored in the fault memory.






7.8 Rear oxygen sensor monitoring - Oscillation Check

7.8.1 General Description
The monitoring function checks the functionality of LSF-type binary oxygen sensors located downstream of the catalyst. The function checks whether the oxygen sensor can reach the voltage thresholds that are necessary for proper operation of exhaust gas control functions and diagnostic functions within the system. This primarily means checking if the setpoint voltage for the second control loop or, additionally, the setpoint voltage for the third control loop as well (SULEV concepts with three oxygen sensors for each exhaust bank), can be reached.

The monitoring function can monitor either a second sensor or a second and third sensor for each exhaust bank.

7.8.2 Rear oxygen sensor, Control threshold not exceeded from lean to rich (stuck lean), P2270/P2272

Master-ECM: Bank 1, Oxygen Sensor 2 P2270 Bank 1, Oxygen Sensor 3 P2274
Bank 2, Oxygen Sensor 2 P2272 Bank 2, Oxygen Sensor 3 P2276
Slave-ECM: Bank 1, Oxygen Sensor 2 P118C Bank 2, Oxygen Sensor 2 P118E

Monitoring Strategy
Initially, the monitor observes if the voltage of the rear oxygen sensors oscillates around the setpoint control voltage of the second (third) control loop with the control being active. If the sensor voltage has exceeded this control threshold from lean to rich and fallen below this threshold from rich to lean, the oxygen sensor is properly functioning.

If the control setpoint threshold is not exceeded from lean to rich even though the proportional part of the rear control has been active for a defined time, an active lambda modification is carried out in order to check if the oxygen sensor voltage can exceed the control threshold.

Typical Enable Conditions
- Intake air mass flow in a defined range
- Modeled exhaust gas temperature is greater than a defined threshold value
- Operational readiness time of the rear oxygen sensor is greater than a defined threshold value
- Second lambda control loop closed

Malfunction Criteria
A fault is suspected if the voltage threshold value for the setpoint of the rear lambda control has not been exceeded after one of the following conditions was fulfilled for the rear control:
- the cumulative activity period is greater than a threshold value

OR
- the integrated oxygen mass flow is greater than a threshold value

To confirm the suspected fault, a rich setpoint lambda value is set with the first control loop being active (during gasoline operation, the enrichment is typically approx. 20 - 30 %). If the setpoint threshold value for this lambda modification is reached and the value remains there for a brief time period and the oxygen sensor voltage still has not exceeded the control setpoint threshold, the fault "control threshold from lean to rich not exceeded (stuck lean)" is set.






7.8.3 Rear oxygen sensor, Control threshold not exceeded from rich to lean (stuck rich) P2271/P2273

Master-ECM: Bank 1, Oxygen Sensor 2 P2271 Bank 1, Oxygen Sensor 3 P2275
Bank 2, Oxygen Sensor 2 P2273 Bank 2, Oxygen Sensor 3 P2277
Slave-ECM: Bank 1, Oxygen Sensor 2 P118D Bank 2, Oxygen Sensor 2 P118F

Monitoring Strategy
Initially, the monitor observes if the voltage of the rear oxygen sensor oscillates around the setpoint control voltage of the second (third) control loop with the control being active. If the sensor voltage has exceeded this control threshold from lean to rich and fallen below this threshold from rich to lean, the oxygen sensor is properly functioning.
If the sensor voltage has not fallen below the control setpoint threshold from rich to lean even though the proportional part of the rear control has been active for a defined time, an active lambda modification is carried out in order to check if the oxygen sensor voltage can fall below the control threshold.

Typical Enable Conditions
- Intake air mass flow in a defined range
- Modeled exhaust gas temperature is greater than a defined threshold value
- Operational readiness time of the rear oxygen sensor is greater than a defined threshold value
- Second lambda control loop closed

When the enleanment of the fuel mixture does not lead to a monitoring result, an additional enable condition applies for the active check with controlled lambda modification:
- Duration of the deceleration fuel cut-off phase is greater than a defined threshold value

Malfunction Criteria
A fault is suspected if the sensor voltage has not fallen below the voltage threshold for the setpoint of the lambda control of the rear sensor after one of the following conditions was fulfilled for the rear control after the catalyst:
- the cumulative activity period is greater than a threshold value
OR
- the integrated oxygen mass flow is greater than a threshold value

To confirm the suspected fault, a lean setpoint lambda value is set with the first control loop being active (during gasoline operation, the enleanment is typically approx. 10 - 15 %; due to reasons of vehicle driveability a gasoline engine can only be leaned-out to a limited extent without engine-power loss and misfiring). If the setpoint threshold value for this lambda modification is reached and the value remains there for a brief time period and the oxygen sensor voltage still has not fallen below the control setpoint threshold, the fault "control threshold from rich to lean not exceeded (stuck rich)" is set.






7.8.4 In-Use Monitor Performance Ratio

Incrementing the numerator:
In the case of a PASS result for the check of the rear sensor, the longest possible monitoring time for a faulty sensor is simulated. When the operational readiness of the oxygen sensor is detected and the second control loop is active, the time required until the active check is started is simulated. For this purpose, a time counter runs until its value exceeds a threshold value. During this time, the sensor voltage is expected to have reached its target value through the control action. When this first time period has elapsed, the active check is simulated by a lambda modification towards rich (for spark-ignition engines, the adjustment to rich is greater than the adjustment to lean, since the rich misfire limit is reached later than the lean misfire limit). For this purpose, the time for the integration ramps (control increase of the lambda request) and the monitoring time are simulated. This time counter is activated when the enable conditions for the lambda modification are fulfilled.

Incrementing the denominator:
The denominator is incremented if the conditions for incrementing the General Denominator according to CCR (d) (4.3.2.) are fulfilled.






7.8.5 Lambda Correction of the Middle Oxygen Sensor, Control Threshold reached, P117A, P117B
Bank 1 P117A
Bank 2 P117B

Monitoring Strategy
The adaptation value of the third control loop is monitored in the same manner as the adaptation value of the second control loop. The adaptation value corresponds to the integral part of the controller in control loop 3. The adaptation value is continuously updated whenever the integral part of the third control loop is activated.

Typical Enable Conditions (Details see Summary Table)
- Integral part of the third control loop has been active for a defined time (Precondition: enable conditions for third control loop are fulfilled)

Enable conditions for the third control loop

General enable conditions:
- The third oxygen sensor, which is located downstream of the total catalyst volume is ready for operation
- No fault from the purge valve monitor is currently present
- No fault from the camshaft monitor is currently present

Additional enable conditions for the proportional part:
- Evaporative control system monitor not active
- Engine coolant temperature > threshold
- Lambda setpoint value = 1
- Catalyst model temperature for the proportional part in a defined range
- Enable delay time after activation of the natural frequency control downstream of the front catalyst (second control loop) has expired

Additional enable conditions for the integral part:
- Fuel rate during EVAP canister purge < threshold
- Engine speed in a defined range
- Engine load in a defined range with the upper and lower thresholds each taken from engine-speed dependent characteristic curves
- Catalyst model temperature for the integral part in a defined range

Malfunction Criteria
If the absolute value of the adaptation value exceeds a fault threshold, a fault of the fuel supply system is set. (No differentiation of the fault is carried out in terms of whether the control threshold is exceeded from rich to lean or from lean to rich, hence a separation of "system too rich" or "system too lean" is not possible.)




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