Part 5
7 EXHAUST GAS SENSOR MONITORING Contd.7.9 Signal Range and Electrical Circuit Monitoring of the Secondary Oxygen Sensors
This diagnostic monitors the second and the third sensor of bank 1.
7.9.1 System overview
Both the downstream oxygen sensor of the precatalyst and the downstream oxygen sensor of the main catalyst are monitored.
7.9.2 General description
The diagnostic monitors the two-step oxygen sensors in the exhaust system for wiring faults and effects of heater switching couplings onto the sensor signal. Short circuits to ground and to battery, and open circuits in the signal wiring and the ground wiring, are detected. If a fault is detected in a sensor, the operational readiness will be withdrawn.
The sensor voltage range for a faultless sensor is 0 to 1 Volt. As it is not possible to measure negative sensor voltages in the control unit, signal range exceedances can only be detected for voltages greater than 1 Volt. Since the sensor voltage for open circuits and shorts to ground remains within the plausible range, an additional evaluation criterion is required: the sensor voltage charged with a load impulse is measured, as it is also utilized to measure the internal resistance of a sensor.
7.9.2.1 General enable conditions (for all the following diagnostics)
Physical Enable Conditions
- Speed for running engine detected (typical speed > 80 rpm)
- Vehicle system voltage > threshold value (typically 11 V)
Enable Conditions for "Operational Readiness of the Oxygen Sensor fulfilled" (to cover the case that the sensor is faultless):
- The sensor voltage has exceeded the cold sensor range of +/- 450 mV (typically lower than 400 mV, or greater than 600 mV but lower than 1080 mV)
- see illustration below
- Sensor is sufficiently heated. This criterion is defined as
- Sensor is sufficiently heated up AND
- Dew point in the vicinity of the sensor is exceeded (can optionally be calibrated)
OR
Enable Conditions for "Theoretical Operational Readiness of Oxygen Sensor fulfilled" (to cover the case that the sensor voltage does not exceed the voltage range due to an open circuit):
- The sensor is sufficiently heated for an extended defined time period AND
- The dew point in the vicinity of the sensor is exceeded (can optionally be calibrated)
Definition of a Sufficiently Heated Sensor
- The modeled exhaust gas temperature in the vicinity of the oxygen sensor exceeds a threshold value
OR
- The heating power exceeds a threshold value for a defined time period
AND
- No heater output stage fault is present in the current driving cycle (monitor is activated immediately after the sensor heating is activated)
7.9.2.2 Short circuit to battery, P0138/P0144
Additional enable condition for this diagnostic:
- Setpoint lambda value > 0.995
Monitoring strategy
Measurement and Evaluation of the Sensor Voltage
Malfunction criteria
In order to prevent sensor voltage peaks > 1 V caused by heater couplings, the diagnostic is not activated before the sensor has sufficiently heated up. These heater couplings occur quite frequently when two-point sensors with a planar sensor element are used.
The condition "sensor sufficiently heated" is set TRUE, when either the heating power exceeds a defined heating power threshold value or the modeled exhaust gas temperature exceeds a temperature threshold value for a defined time period, and when the completed heater output stage diagnostic has not detected a fault in the current driving cycle.
When, during sensor operation between slightly rich (lambda > 0.995) and lean, a voltage is detected that exceeds a threshold value for a defined time period (typically 1.08 V), the fault P0138 for the sensor 2 of bank 1, or the fault P0144 for the sensor 3 of bank 1, is set, and the operational readiness of the sensor is withdrawn.
7.9.2.3 Short circuit to ground, P0137/P0143
Additional enable conditions for this diagnostic:
- Overrun fuel cut-off inactive
- Catalyst purge inactive (after overrun phase)
Monitoring strategy
Measurement and evaluation of the sensor voltage, when charged with load pulses
Malfunction criteria
When the sensor voltage remains below a minimum threshold value (typically 40 to 60 mV) for a defined time period (typically 3 s), and if during this period neither overrun fuel cut-off nor catalyst purge after overrun is active, a fault is indicated and the corresponding flag is set. This fault indication triggers the request of three consecutive load pulses. For each of these load pulses, the unloaded and the loaded sensor voltages are measured.
When the mean value of the differences between the loaded and the unloaded sensor voltages is lower than a minimum calibratable threshold value, the fault P0137 for the sensor 2 of bank 1 or the fault P0143 for the sensor 3 of bank 1 is set, and the operational readiness of the sensor is withdrawn.
The measurement with load pulses corresponds to the measurement of the sensor internal resistance, as it is utilized for determining the sensor temperature, and for the heater diagnostic.
Determination of the Loaded Sensor Voltage, the Unloaded Sensor Voltage, and the Internal Resistance
The internal resistance of the oxygen sensor's Nernst cell can be determined by using a pulsed pump current. Since the internal resistance increases substantially when the heater is defective, or when the heating power is considerably decreased, with the exhaust temperature not being too high (typically T < 700 °C), the internal resistance is used, indirectly, to monitor the sensor heating. Additionally, the internal resistance may 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.
7.9.2.4 Signal Wire or Ground Wire, Open Circuit - P0140/P0146
Monitoring strategy
Due to the wiring arrangement in the ECU (high-resistance connection of the sensor to a countervoltage of 450 mV), exceedances of the +/- 450 mV voltage range indicate that the internal resistance of the Nernst cell has exceeded a defined threshold. Therefore, these exceedances of the above voltage range are also utilized to detect the operational readiness of the oxygen sensor.
Depending on the current exhaust gas composition in the exhaust system, the oxygen sensor voltage drops below the minimum (lean) voltage threshold value (typically 400 mV), or it exceeds the maximum (rich) threshold value (typically 600 mV), however remaining below the short circuit voltage threshold value (typically 1.08 V).
Malfunction criteria - signal wire, open circuit
When the sensor voltage remains within the voltage range of +/- 450 mV, the oxygen sensor is either too cold, or the signal wire is interrupted.
When the sensor voltage remains within the voltage range of approximately 400 to 600 mV after the sensor has reached its operational readiness, or the sensor has reached its theoretical operational readiness for a defined time period (typically 3 s), a fault is indicated.
This fault indication triggers the request of three consecutive load pulses. For each of these load pulses, the unloaded and the loaded sensor voltages are measured.
When the mean value of the differences between the loaded and the unloaded sensor voltages is greater than or equal to a threshold value, the fault P0140 for the sensor 2 of bank 1 or the fault P0146 for the sensor 3 of bank 1 is set, and the operational readiness of the sensor is withdrawn.
Malfunction criteria - ground wire, open circuit
Additional enable condition for this diagnostic:
- To ensure that there is a valid measurement of the sensor internal resistance, the number of internal resistance measurements must exceed a threshold value (typically 6 to 10).
Using planar sensors, heater couplings onto the sensor signal may occur at high exhaust temperatures and an interrupted ground wire. Therefore the voltage does not necessarily have to remain within the +/- 450 mV range.
For this reason, another diagnostic path exists to reliably detect an interrupted ground wire. When the internal resistance exceeds a maximum threshold value (typically > 40.000 Ohms) and the modeled exhaust gas temperature exceeds a maximum threshold value (typically 660 °C), the fault P0140 for the sensor 2 of bank 1 or the fault P0146 for the sensor 3 of bank 1 is set, and the operational readiness of the sensor is withdrawn.
A fault must be assumed if the sensor voltage does not exceed the voltage range defined for a cold sensor, despite sufficient heating, for an extended time period (approx. 20 s), even if no open circuit fault has been detected by the current-pulse method. In this instance, the operational readiness is forced. This is to enable fault detection by the oscillation monitoring of the secondary oxygen sensor and, consequently, a "stuck lean" fault is registered. Hence, it is even possible to detect a manipulation by simulating the presence of the sensor with a fixed voltage between 400 mV and 500 mV applied to the sensor signal pin.
7.9.3 In-Use Monitoring Performance Ratio
Since all above diagnostics are continuous monitors, neither numerators nor denominators are created.
7.10 Rear O2 Sensor response monitoring 7.10.1
General Description
A second oxygen sensor is located downstream of the catalyst in the exhaust system. This secondary oxygen sensor measures the oxygen concentration downstream of the catalyst. This way, the oxygen concentration in the exhaust gas upstream of the catalyst can be compared to the oxygen concentration downstream of the catalyst, which serves to monitor the catalyst's oxygen storage capacity.
In exhaust systems with divided catalysts or two catalysts one or two oxygen sensors may be installed downstream of the catalysts. The first oxygen sensor is typically installed downstream of the first partial catalyst volume and is used for the catalyst monitoring within the second control loop of the lambda control. The second oxygen sensor, which is used for SULEV concepts, is installed downstream of the total catalyst volume and is used both in the third control loop of the lambda control and, depending on the concept, in the catalyst monitor for the monitoring of the total catalyst volume. The switch-over to the total catalyst volume monitoring is carried out only after the first catalyst has reached a defined state of aging.
Aging, contamination or insufficient heating of the secondary (third) oxygen sensor may slow down the sensor's dynamic behavior. A slow response rate or a long transition time could influence the catalyst diagnostic and can lead to higher emissions.
The dynamic behavior monitoring serves to evaluate the response behavior of an LSF-type secondary oxygen sensor during deceleration fuel cut-off phases, i. e. when a distinct fuel mixture surge from rich to lean occurs. A faulty sensor or a sensor that is deteriorated can be detected by determining and evaluating the transition time and the response time.
The dynamic behavior monitoring is designed to be executed repeatedly (multiple times) within the same driving cycle as long as the enable conditions are fulfilled or fulfilled again.
Due to monitoring during deceleration fuel cut-off phases, typical monitoring frequency during a FTP72 is at minimum one to two events, during the Unified Cycle approximately 5 events and during normal driving on road in customer's hand the typical frequency is 10 to 15 events (based on engineering evaluation of road-data).
Diagnostic Enable / Disable Conditions
Physical Parameters:
- HO2S ready
- Exhaust temperature post catalyst above an applicable level (typical value: 500°C) (dew point exceeded; to ensure homogeneous temperature distribution in the catalyst)
- Internal resistance of the HO2S is less than an applicable value
- HO2S voltage able to cross control level (typical value: 0.6-0.65 V) of fuel trim (optional parameter)
- Exhaust mass flow exceeds an applicable value (typical value: ~15-25 kg/h)
- Slope of exhaust mass flow post catalyst is less than an applicable value ((typical value: Delta mass flow of ~10 kg/h)
Disable Conditions:
- Cycle flag of aging diagnostic HO2S post catalyst (lash) not set or failure(s) detected
- Cycle flag of EVAP purge diagnostic not set or failure(s) detected
- Cycle flag HO2S front catalyst not set or failure(s) detected
- No failures detected on final stage for O2S heater
Signal Evaluation / Validation:
For a robust monitoring function the HO2S signal needs to fulfill the following conditions ensuring a certain level of signal quality:
- HO2S voltage post catalyst exceeds a certain level (typical value: ~0.55-0.6 V) before fuel cut-off occurs
- Fuel cut-off is present (Bit for this condition is set)
- HO2S in front of the catalyst is ready and shows definitely "lean" (e.g. Lambda is equal or greater than 10). The front HO2S signal range maximum value is equivalent to a Lambda of 16.
- The signal of the HO2S is monotone decreasing (outlier detection)*
- No HO2S faults detected by the dynamic diagnostic within the actual driving cycle
Outlier Detection
The outlier detection is necessary to exclude interferences of the catalyst during fuel cut-off conditions on the HO2S signal.
The rich-to-lean dynamic diagnostic of the H02S post catalyst is based on the physical / chemical effect that during fuel cut-off the Lambda value changes in a step response from Lambda equal one to a Lambda equal or greater than 10 indicating the Oxygen storage capability (OSC) of the catalyst is exceeded. In reality the signal behavior of the rear O2S shows occasionally during fuel cut-off a first decrease of sensor voltage to approximately 0.4 Volts, remains at that voltage level for a certain time and then falls again to a voltage of 0.1 Volts. In this specific case the measured transition time is above the fault threshold of the diagnostic but can not be counted as a O2S failure because the event is based on chemical reaction within the catalyst.
Explanation of chemical effect:
At the beginning of the fuel cut-off the CO post catalyst reaches a minimum level because of the increase of Oxygen available in the catalyst allowing an increase in CO-conversion rate into CO2, the O2S voltage decreases to approximately 0.4 Volts and stays at that level until the OSC of the catalyst is exceeded and the Oxygen level post catalyst increases causing another O2S voltage drop to approximately 0.1 Volts. This interference of the catalyst on the O2S voltage must be excluded from further evaluation by the monitor to prevent false failure detection by the response time diagnostic.
The described physical / chemical effect on the rear O2S signal is shown in figure below.
7.10.2 Transition time of the sensor too long, P0139, P0145 (DDYLSH, DDYLSF)
P0139: Secondary oxygen sensor for systems with two oxygen sensors
P0145: Secondary oxygen sensor for systems with three oxygen sensors
Applicable only for test group:
Monitoring Strategy
Following a mixture surge from rich to lean, the time it takes for the sensor voltage to drop from one threshold value to another value is evaluated.
Typical Enable Conditions (Details see Summary Table)
- Sensor voltage ≥ defined threshold value
- Signal of primary oxygen sensor > defined threshold value
- Modeled exhaust gas temperature > defined threshold value
- Gradient of exhaust gas mass flow < defined threshold value
- Internal resistance of secondary sensor ≤ defined threshold value
Malfunction Criteria
The monitor is started when deceleration fuel cut-off is active and when all enable conditions are fulfilled. At the start of a deceleration fuel cut-off phase the oxygen contained in the exhaust gas is initially absorbed by the catalyst. When oxygen can no longer be stored in the catalyst, the oxygen concentration at the secondary oxygen sensor increases abruptly, causing the sensor voltage to start to drop.
As the sensor voltage decreases, the transition time is measured during which the sensor voltage drops from one threshold value to another threshold value. In each monitoring cycle during which no oxygen sensor surge is detected, the currently determined transition time is used in a EWMA-filter. The EWMA-filter is described in a separate chapter. When the EWMA-filter has stabilized after a defined number of measurement values have been added to it and the filter value exceeds a defined threshold value, a sensor is detected that is too slow, and a fault is stored.
In order to detect sensor surges, the difference between the transition time and the filter value of the EWMA-filter is generated. If the absolute value of this difference is greater than a threshold value, a sensor surge is detected and a fault counter is incremented. After a defined number of monitoring events an arithmetic mean value is generated from the transition times calculated after the sensor surge. If the arithmetic mean value is greater than a defined threshold value, a sensor transition time is detected that is too long, and a fault is stored.
Due to a chemical effect in the catalyst, the sensor voltage can sometimes initially decrease to a value of approx. 0.4 V after a deceleration fuel cut-off is started, and then continue to decrease, but only after a short dwell time. The reason for the first voltage drop is the high conversion rate of carbon monoxide to carbon dioxide that occurs at the start of the deceleration fuel cut-off phase when a lot of oxygen is present in the catalyst. The second voltage drop occurs with a delay to the first drop, when the catalyst is saturated with oxygen. The outlier detection detects such a voltage curve and prevents false fault detection due to this voltage curve, as in this case a transition time which is too long must not be diagnosed.
7.10.3 Response time of the sensor too long, P2271, P2273
Sensor 2: bank 1: P2271 (Master) and bank 3: P118D (Slave)
Sensor 2: bank 2: P2273 (Master) and bank 4: P118F (Slave)
Sensor 3: bank 1: P2275
Monitoring strategy
Following a mixture surge from rich to lean, the sensor voltage is evaluated when a defined oxygen mass has been supplied to the catalyst.
Typical Enable Conditions
- Time from start of deceleration fuel cut-off ≥ defined threshold value
- Duration of deceleration fuel cut-off ≤ defined threshold value
- Exhaust gas temperature ≥ defined threshold value
- Exhaust gas mass flow > defined threshold value
- Change in exhaust gas mass flow within a defined range
- Sensor voltage > defined threshold value
- Actual lambda value upstream of the catalyst is within a defined range around the setpoint value of the lambda upstream of the catalyst
Malfunction Criteria
At the start of a deceleration fuel cut-off phase the oxygen contained in the exhaust gas is absorbed by the catalyst. When oxygen can no longer be stored in the catalyst, the oxygen concentration at the secondary oxygen sensor increases abruptly, causing the sensor voltage to start to drop. The creation of an oxygen mass flow integral is already started when the deceleration fuel cut-off is activated.
The monitor starts when, beginning from the activation of the deceleration fuel cut-off the catalyst has been charged with oxygen mass to a defined level, and if all other enable conditions are fulfilled.
The diagnostic monitors the oxygen mass flow integral and in this way the oxygen mass supplied to the catalyst. When the oxygen mass has reached a defined threshold value that corresponds to the maximum oxygen storage capacity of the catalyst, the current sensor voltage is compared to a defined threshold value. If the sensor voltage is not below a defined threshold value, a response of the oxygen sensor is detected which is too slow, and a fault is registered.
7.10.4 In-Use Monitor Performance Ratio
Incrementing the numerator:
The numerator is incremented once per driving cycle when a fault has been detected, or when a defined number of monitors have been carried out completely in which the mass air flow integral has exceeded a defined threshold value during the deceleration fuel cut-off phase.
Incrementing the denominator:
The denominator is incremented by one if the conditions for incrementing the General Denominator according to CCR (d) (4.3.2.) are fulfilled.