Part 2

7 EXHAUST GAS SENSOR MONITORING Contd.

Case 5: Fail result due to an asymmetrical slow transition fault
If the difference between the dynamic values for the R2L flank and the L2R flank is less than a minimum threshold value or greater than a maximum threshold value, an asymmetrical fault is detected.
- If the higher one of the two counters (for the R2L flank and the L2R flank) exceeds a threshold value Z1 (typically 4 to 6)

and

- the corresponding gradient ratio (for the R2L flank or the L2R flank) exceeds a lower threshold value G1

and

- the corresponding gradient ratio (for the R2L flank or the L2R flank) falls below a second, higher threshold value G2

and

- the corresponding area ratio (for the R2L flank and the L2R flank) falls below the threshold value for the asymmetrical transition fault D4

or

- the corresponding dynamic value for the gradient ratio falls below the threshold value G1,

the sensor is dynamically defective, the monitor is ended and a dynamic fault is stored.






In-Use Monitor Performance Ratio

The incrementing of the numerator and the denominator, and the ratio calculation for the response rate monitor of the primary heated oxygen sensor are executed by the IUMPR kernel function. In accordance with all monitors requiring a standardized track and report of the in-use performance, the rate response monitor of the oxygen sensor reports to the IUMPR kernel function via status flags - see the description of the IUMPR kernel function.

Incrementing the numerator
Measurements with a properly working oxygen sensor can usually be taken quickly and without any disturbances. A slow oxygen sensor increases the measuring time only insignificantly. The measurements, however, are interrupted more frequently. Therefore, the time required for fault detection and hence for incrementing the numerator has to be simulated by a shadow counter.
A time counter is started, whenever the enable conditions and the conditions for the signal evaluation/validation are fulfilled, and when the waiting period between two fuel surges has elapsed. This time counter is repeatedly increased each 100 ms, until a calibratable threshold value is exceeded. This fault-case measuring time must be adapted to the worst case fault. When this time period for one event has elapsed, an event counter is incremented. When the event counter exceeds a threshold value in an error-free system, the condition "numerator complete" is set.
The numerator for the sensor dynamic diagnostic is also incremented, when an actual fault is detected.

Incrementing the denominator
The denominator is incremented, when the standardized conditions during a current driving cycle, in accordance with CCR (d) (4.3.2.) (E) (ii), are fulfilled, and when the monitoring function is not affected or inhibited by already stored faults or faults of sensors that are necessary to determine the denominator.


7.2 Voltage Signal Monitoring of the Front Oxygen Sensor

7.2.1 General Description
This monitoring function checks if the voltage of the amplified output signal from the front heated wide range oxygen sensor is in a defined range. At stoichiometric (X approximately = 1) or slightly lean engine operation, the output voltage of the oxygen sensor is generally considerably lower than the measured voltage value of an oxygen sensor that is surrounded by ambient air. A fault can be caused, for example, by an active oxygen sensor that is not installed correctly in the exhaust system.






7.2.2 Leakage Air in the Exhaust System - upstream of the Catalyst, P2414, P2415
Master: Bank 1, sensor 1: P2414 Bank 2, sensor 1: P2415
Slave: Bank 3, sensor 1: P118A Bank 4, sensor 1: P118B

Monitoring Strategy
The monitor checks if the oxygen sensor voltage is in a range that corresponds to an oxygen concentration which is too high.

Typical Enable Conditions (Details see Summary Table)
- Lambda setpoint value < threshold value
- Temperature of the oxygen sensor ceramic > threshold value
- Deceleration fuel cut-off not active
- Oxygen sensor heater control active
- Secondary air injection not active

Malfunction Criteria
If the oxygen sensor voltage is in a defined fault range for a defined time period (fault confirmation time), the fault "Leakage air in the exhaust system upstream of the catalyst" is detected and a fault is stored in the fault memory. At low fuel levels, a longer fault confirmation time is used. This is done to avoid fault entries because of enleanments due to low residual fuel amounts. Depending on the current gain factor, which depends, in turn, on the X value, different fault ranges are used accordingly. Around X = 1 (typically in the range of 0.95 < X < 1.05), a high gain factor (typically V = 17) is used. For a rich or lean fuel mixture a low gain factor (typically V = 8) is used.
The upper fault range threshold value can only be exceeded if the trim circuit is faulty. For this reason, the oxygen sensor voltage during deceleration fuel cut-off is compared to this threshold value for the detection of an open trim circuit within the circuit monitoring function.






In-Use Monitor Performance Ratio

Incrementing the numerator
The numerator is incremented when a fault has been detected or when the sensor voltage is in the permitted range below the lower fault range threshold value for a defined time period and the fuel level is validated. If the fuel level is low, the sensor voltage must be in the permitted range for the longer defined fault confirmation time.

Incrementing the denominator
The denominator is incremented when the conditions for incrementing the general denominator according to CCR (d) (4.3.2) (B) are fulfilled.

7.3 Circuit Monitoring Of Wide Range Oxygen Sensor (LSU)

7.3.1 General Description
The wide range oxygen sensor (LSU) is used with a signal processor IC for continuous lambda control. The LSU contains a Nernst cell and a pump cell. The lambda value in the Nernst cell is continually adjusted to lambda = 1 independently of the lambda value (oxygen part) at the exhaust gas side by a current through the pump cell. The control and evaluation of the current, as well as the monitoring function are carried out in the signal processor IC.

A fault is generally set if the monitoring function detects short circuits or voltages that are too low, or if the sensor measures irrational voltage values.






Lambda control can be disabled only when at least a pending fault code (service $07) has been entered in the fault code memory. Several operating conditions exist, under which faulty components can be pinpointed only after a time delay. Lambda control must however be disabled when fault symptoms are detected.

The diagnostic function detects and sets a general electrical fault. The actual fault which is accompanied by a second entry into the fault code memory is then pinpointed afterwards by the appropriate diagnostic function. This second fault entry could either be a sensor line error or a heater fault of the upstream O2 sensor. In order to ensure timely MIL illumination (two-in-a-row principle) the general electrical fault is also set when a sensor line error or a heater fault of the upstream O2 sensor is set.

A general electrical fault of the upstream heated oxygen sensor will be set either
- when the internal resistance of the O2 sensor's Nernst cell is implausibly high or the ceramic temperature of the FO2 sensor is implausibly low. All of the following monitoring conditions must be fulfilled in both cases:
- the exhaust gas temperature model is valid
- the modeled exhaust gas temperature at the upstream O2 sensor's location lies above a calibrated minimum
- the FO2 sensor's heater is enabled/switched on and throttle fuel cut-off condition doesn't exist
- the FO2 sensor's heater diagnostic has not been inhibited (battery voltage supply)
- no errors from the FO2 sensor's heater power stage
- no faults of the FO2 sensor's evaluation IC
- no short circuits of the FO2 sensor
- or when the FO2 sensor's IC Nernst voltage line diagnostic is performed via its IC diagnostic and an interruption of this line is detected.
- or when the FO2 sensor's virtual ground line diagnostic has been performed via its IC diagnostic and an interruption of this line is detected
- or when the FO2 sensor's heater circuit diagnostic is performed and a heater fault is detected.






7.3.2 Short to Ground, P0131/P0151 and Short to Battery P0132/P0152
Short to Ground Master: Bank 1: P0131 Bank 2: P0151
Short to Ground Slave: Bank 1: P3205 Bank 2: P3235

Short to Battery Master: Bank 1: P0132 Bank 2: P0152
Short to Battery Slave: Bank 1: P3206 Bank 2: P3236

Monitoring Strategy
For Short to Ground:
The voltages in the signal circuits are compared to lower threshold values.
For Short to Battery:
The voltages of the signal circuits are compared to upper threshold values.

Typical Enable Conditions (Details see Summary Table)
- Output stage of signal processor IC is activated

Malfunction Criteria
For Short to Ground:
If the Nernst voltage or the voltage of the trim circuit or pump circuit fall below the respective lower threshold value, or the voltage of the "virtual ground" signal circuit is below a lower threshold value, a fault is detected.

For Short to Battery:
If the Nernst voltage or the voltage of the trim circuit or pump circuit exceeds an upper threshold value, or the voltage of the "virtual ground" signal circuit is above an upper threshold value, a fault is detected.

7.3.3 Open Circuit in VM Signal Circuit ("Virtual Ground"), P2251/P2254
Master: Bank 1: P2251 Bank 2: P2254
Slave: Bank 1: P3285 Bank 2: P3295

Monitoring Strategy
Within a defined oxygen sensor voltage range the internal resistance is compared to a threshold value.

Typical Enable Conditions (Details see Summary Table)
- Modeled exhaust gas temperature < threshold value
- No deceleration fuel cut-off for a defined period of time
- Heater control active
- Defined time period since the end of the heater start-up ramp has elapsed
- No pump current switch-off active
- No heater output stage fault is currently present
- The results of the internal resistance measuring are valid
- Vehicle system voltage > 11 V
- The heater is switched on for a minimum time period after the following events:
- Switching the heater control off
- Engine speed is less than (minimum) starting engine speed
- Voltage drop beyond 11 V
- Deceleration fuel cut-off
- The 3 kHz measurement pulse that is used to measure the internal resistance is switched off for at least 2 s

Malfunction Criteria
An open circuit in the VM sensor circuit results in a sensor voltage of approx. 1.5 V and to an irrationally high internal resistance of the sensor. The heater control responds to the high internal resistance by increasing the heating output to its maximum value. The actual ceramic temperature no longer correlates with the ceramic temperature calculated from the internal resistance. For this reason the duty cycle of the heater control is limited if a fault is detected, in order to prevent the sensor from being damaged.
In order to measure the sensor voltage, the 3 kHz measurement pulse that is used to measure the internal resistance is disabled. In the case of an open circuit, the voltage then remains within a narrow range around 1.5 V.

7.3.4 Open Circuit in UN Signal Circuit (Nernst voltage), P2243/P2247
Master: Bank 1: P2243 Bank 2: P2247
Slave: Bank 1: P3281 Bank 2: P3291

Monitoring Strategy
If the oxygen sensor voltage is outside a defined range the internal resistance is compared to an upper threshold value.

Typical Enable Conditions (Details see Summary Table)
- Heater control active
- Defined time period since the end of the heater start-up ramp has elapsed
- No pump current switch-off active
- Currently no heater output stage present
- The 3 kHz measurement pulse that is used to measure the internal resistance is switched off for at least 2s

Malfunction Criteria
An open circuit in the UN sensor circuit results in an irrational sensor voltage and to an irrationally high internal resistance of the sensor. The heater control responds to the high internal resistance by increasing the heating output to its maximum value. The actual ceramic temperature no longer correlates with the ceramic temperature that is calculated from the internal resistance. For this reason the duty cycle of the heater control is limited when a fault is detected, in order to prevent the sensor from being damaged.
In order to measure the sensor voltage, the 3 kHz measurement pulse that is used to measure the internal resistance is disabled. In the case of an open circuit, the voltage then approaches either 0 V or 5 V.


7.3.5 Open Circuit in IA Signal Circuit (Trim Circuit), P2626/P2629
Master: Bank 1: P2626 Bank 2: P2629
Slave: Bank 1: P3278 Bank 2: P3288

Monitoring Strategy
The oxygen sensor voltage is compared to an upper threshold value.

Typical Enable Conditions (Details see Summary Table)
- Modeled exhaust gas temperature < threshold value
- Temperature of the oxygen sensor ceramic > threshold value
- Deceleration fuel cut-off active
- Heater control loop closed
- No pump current switch-off active
- Valve of the external exhaust gas recirculation is closed (only for systems with external EGR)

Malfunction Criteria
The trim circuit (IA) from the sensor to the IC and the trim resistor that is integrated in the sensor connector ensure the correct characteristic slope of the LSU. If an open circuit in the trim circuit occurs, the slope of the characteristic curve becomes irrational and the sensor voltage is too high during deceleration fuel cut-off, when the sensor is only supplied with air. During lambda =1 operation this error is not noticeable, as the pump current approaches zero. If the oxygen sensor voltage exceeds the upper threshold value for a minimum time period, an open circuit in the trim circuit is detected. The fault threshold is switched depending on the current signal amplification (amplification factor V=8 or V=17).






7.3.6 Open Circuit in IP (Pump Circuit) Signal Circuit, P2237/P2240
Master: Bank 1: P2237 Bank 2: P2240
Slave: Bank 1: P3278 Bank 2: P3288

The pump current circuit provides the LSU with the pump current that is required to adjust a voltage of 450 mV in the Nernst cell. In this case, the pump current is equivalent to the oxygen concentration in the exhaust gas. If this circuit is open, the pump current is permanently zero and the output voltage of the sensor is 1.5 V, independent of the oxygen concentration in the exhaust gas.
There are two ways to detect an open circuit in the pump current circuit:
a. Rationality check of the control action (preferred method)
b. Rationality check of the sensor voltage during deceleration fuel cut-off (optional method; additionally activated in the corresponding engine variants)

7.3.6.1 Rationality Check of the Lambda Control Action

Monitoring Strategy
If the oxygen sensor voltage is within a narrow voltage range around 1.5 V, the change in lambda control is observed or an active check is started.

Typical Enable Conditions (Details see Summary Table)
- Temperature of the oxygen sensor ceramic > threshold value
- Defined time period since the end of the heater start-up ramp has elapsed
- No pump current switch-off active
- No switching on and off of the P-part of the second control loop
- No switching on and off of catalyst heating measures
- Electrical adaptation of the sensor IC not active
- Lambda modulation > threshold value, i. e. lambda value has been valid for defined period
- Lambda control loop closed
- Heater control loop closed

Malfunction Criteria
If the sensor voltage is within a defined voltage range (i. e. the sensor voltage is greater than a lower threshold value and less than an upper threshold value), when the forced modulation of the lambda value is greater than a minimum threshold for at least a defined minimum time period, OR when the lambda setpoint value has been switched to a rich value for at least a defined minimum time period and the change in lambda control output value is greater than a threshold value or the lambda control value is at its upper or lower limit, an open circuit in the pump circuit is detected.






7.3.6.2 Rationality Check of the Sensor Voltage during Deceleration Fuel Cut-off

Applicable only for test groups:






Monitoring Strategy
The sensor voltage is compared to a lower threshold value during the deceleration fuel cut-off.

Typical Enable Conditions (Details see Summary Table)
- Temperature of the oxygen sensor ceramic > threshold value
- Electrical adaptation not active
- Heater control loop closed

Malfunction Criteria
If deceleration fuel cut-off has been active for a certain period of time and the oxygen sensor voltage falls below a threshold value, an open circuit in the pump circuit is detected.






7.3.7 Communication Test via Number of Message Errors, P0606
Master: Bank 1/2: P0606
Slave: Bank 1/2: P0606

Monitoring Strategy
The number of message errors is compared to an upper threshold value.

Typical Enable Conditions (Details see Summary Table)
- Battery voltage within a voltage range
- Engine speed > threshold value

Malfunction Criteria
The signal processor IC communicates with the main processor of the control module via an SPI bus. If the SPI bus is disrupted, resulting in irrational commands, a fault counter is incremented and the last command is sent again. An IC communication error is detected if an upper threshold value is exceeded.

7.3.8 Communication Test via Rationality Check of the Initialization Registry, P0606
Master: Bank 1/2: P0606
Slave: Bank 1/2: P0606

Monitoring Strategy
The initialization registry is compared to a mirror registry.

Typical Enable Conditions (Details see Summary Table)
- Battery voltage within a voltage range
- Engine speed > threshold value

Malfunction Criteria
Whenever the initialization registry is rewritten, after a delay the initialization registry is read and compared to a mirror registry. If the two registry contents do not match, an IC communication error is detected.






7.3.9 Self-test of Sensor IC, IC Supply Voltage Too Low, P0606
Master: Bank 1/2: P0606
Slave: Bank 1/2: P0606

Monitoring Strategy
The supply voltage of the IC is compared to a lower hard-coded threshold value of 10.7 V.

Typical Enable Conditions (Details see Summary Table)
- Battery voltage within a voltage range
- Engine speed > threshold value

Malfunction Criteria
If the supply voltage of the IC remains below a defined threshold value for a defined time period, the corresponding fault is detected and stored in the fault memory.

7.4 Rationality check of the primary catalyst oxygen sensor

7.4.1 General Description
This function is a rationality check of an upstream LSU-type oxygen sensor. The measurement deviation (offset) from the characteristic curve of this oxygen sensor is evaluated. The offset adaptation is carried out within a separate function. The offset value is determined from the signal of the second LSF-type binary oxygen sensor downstream of a catalyst and from the control action of the second lambda control loop. It is used to adjust the measured lambda value of the first (upstream) oxygen sensor in order to neutralize the effects on the exhaust gas emissions, and it is used for monitoring the oxygen sensor and the fuel system.

The function shall detect the following fault types:

- Leakage currents in the pump circuit of the oxygen sensor
These leakage currents distort the measured pump current and, as a result, the lambda signal. As the correlation between the pump current and the lambda value is not linear, the lambda deviation depends on the respective lambda position, i. e. the same leakage current has a smaller effect with a lean mixture, than with a rich mixture.

- Characteristic Shift Down (CSD)
In the case of a CSD, the measuring chamber of the LSU-type oxygen sensor is adjusted to the incorrect lambda value. In the case of a typical CSD with catalytically reductive components this results in a lambda value which is considerably too rich. If oxygen sensors with a pumped reference are used, this fault type occurs only rarely. The only possible reason for this type of fault could then be a reference pump current decrease or breakdown.

- Leakage in the exhaust system between the upstream and the downstream oxygen sensor
This fault pattern is not an oxygen sensor fault. However, the leakage causes an irrationality of the upstream sensor signal and the downstream sensor signal, too, which is the characteristic criteria of an offset.

For offset faults, two types of faults are distinguished: major faults, which are reported as sensor faults of the upstream oxygen sensor, and minor offset faults, which are reported as fuel system faults.

The interaction of the functions
The offset determination is integrated in the structure of the lambda control1 (first and second control loop), as well as in the function that calculates the lambda value.
Using the software structure, the offset adaptation and hence the diagnostic can be largely decoupled from the lambda control, i. e. from an exhaust gas relevant function. In this way, a slow second control loop can be implemented with a fast offset adaptation.

Offset adaptation
The adaptation function has to adapt the measurement deviation (offset) of the LSU-type oxygen sensor by interpreting the control action of the second control loop (fuel trim).

Effect of the offset faults of the LSU-type sensor
An offset has the effect that the measured lambda value deviates from the actual lambda value. A positive offset or lean offset has the effect that the measured lambda is greater (leaner) than the actual lambda. A negative offset or rich offset results in a lambda value which is too low (rich).