Self Diagnosis - Theory & Operation (N54): Overview BMW X6 E71

1.1.1 Diagnostic overview

Catalyst monitoring is based on the monitoring of the o xygen s torage c apability (OSC) by comparing the signals of the O2 sensor upstream and downstream of the catalyst. The engine control stimulates the regular lambda oscillations of the exhaust gas. These oscillations are needed for best possible catalyst conversion. They are damped by the storage activity of the catalyst. The amplitude of the remaining lambda oscillations downstream the catalyst indicates the oxygen storage capability.

The efficiency of the catalyst system is tested during steady state driving by cycling the air fuel ratio LEAN and then RICH for a calibratable number of cycles while monitoring the OSC.

Prior to the catalyst test the canister purge valve is closed or opened with low canister purge value. This is to eliminate the influence of canister vapors on the downstream sensor during the test.

1.6.4.1 General description

The Injection Deactivation is a diagnosis to protect the catalysts from overheating. In fact it is an additional fuel pressure diagnosis with a very short diagnostic time and an early reaction with deactivation of some injectors. Additional there is an evaluation of the fuel tank level. If the tank level is low there will be a change of the DTC to provide better information's for the workshop.

Scheme 51

The diagnosis evaluates first an air-fuel deviation. This check analyses the difference between the measured air fuel ratio and the set-point air fuel ratio and analyses the short term fuel trim. Both checks detect only the lean side of mixture. If a lean deviation occurs the diagnosis compares the difference between measured pressure and the setpoint of the high pressure and the absolute value of the high pressure sensor with a threshold or checks the absolute value of the low pressure sensor. Then a DTC is set and the reaction is the shut-off of an amount of injectors. This reaction avoids overheating of the catalysts. The decision of which DTC is stored will be affected by the fuel tank level ( 0 litre).

1.6.7.1 Monitoring overview

The diagnosis of the fuel level sensor signal consists of a circuit continuity check and a rationality check.

1.7.1.2.2 Monitoring description

This function determines, if an open circuit in any of the four electric lines ( Reference Voltage, Virtual Ground, Pumping Current and Trim Current ) is present in the w ide r ange a ir f uel (WRAF) sensor.

This function shall be triggered only if one of the following diagnosis is active (to set the readiness bit), which are ' UPSTREAM OXYGEN SENSOR - SIGNAL MONITORING DURING FUEL CUT-OFF ' and ' UPSTREAM OXYGEN SENSOR - HEATER MONITORING '. The function shall go to the state = "active" only if one of the above diagnosis detected a fault.

(Reference Voltage)

If a heater error exists and sensor voltage is too low, while the internal resistance measurement is turned off, an open circuit in the line reference voltage occurred. Before the internal resistance measurement is turned off, the sensor temperature-failure P3026/P3027 is stored.

(Virtual Ground) or (Pumping Current)

An open circuit in line virtual ground or in the line pumping current can be detected if the sensor signal stocks near lambda 1. The sensor non-activity can be detected by the Oxygen Sensor Signal Monitoring during fuel cut-off (signal voltage below e.g. 2.1 V) in fuel cut-off).

(Trim Current)

If the sensor shows an augmented gain, i.e. the sensor signal is higher than the nominal characteristic line, the plausibility test during the fuel cut-off phase shall detect this symptom (signal voltage above e.g. 5,6 V) during fuel cut-off) and an open circuit is assigned to the line trim current.

1.7.1.5.1 Monitoring description

This function will detect if the Oxygen Sensor wire harness has been cross connected, i.e., Bank 1 with Bank 2. This is performed by the use of the output of the fuel correction (lambda controller) of each bank. If this control is on opposite limit at bank 1 and bank 2, the sensors are swapped and the corresponding fault code is stored.

Corresponding fault code

P0040

1.7.1.6.1 Monitoring description

This function shall deliver information indicating that the sensor characteristic line has a shift to lean (Characteristic Shift Down) or to rich, which shall be done by summarizing all similar failure symptoms of this kind.

In dependence of the shift strength there are three different paths followed by this diagnosis

1. Strong shift to lean/rich: If the lambda sensor upstream shows a rich signal while downstream lambda sensor signal is lean (or vice versa) and additionally the lambda controller goes to its limit, this error is recognized by the upstream sensor plausibility check.
2. Middle strong shift to lean/rich: If the trim controller goes to its limit but the lambda controller does not, the downstream oxygen sensor signal activity check (P114A, P114B, P114C, P114D) recognizes that the system has a problem and a failure code is stored. Referring to this failure entry, the «DOWNSTREAM ACTIVE TEST»(ref-361402-S41904915162010051400000) is triggered. It detects that the problem is in the upstream oxygen sensor, which is showing a characteristic line shift to lean or to rich. The appropriate DTC will be stored along with the downstream sensor signal activity check DTC.
3. Mild shift to lean/rich: The trim controller I-share goes to its limit but the lambda controller does not. The trim control plausibility monitoring (P2096, P2097, P2098, P2099) recognizes that the system has a problem and a failure code is stored. Referring to this failure entry, the «DOWNSTREAM ACTIVE TEST»(ref-361402-S41904915162010051400000) is triggered. It detects that the problem is in the upstream oxygen sensor, which is showing a characteristic line shift to lean or to rich. The appropriate DTC will be stored along with the fuel correction DTC.

1.7.1.9.1 Diagnostic overview

The purpose of this function is to detect oxygen sensor heater failures that would lead to an increase in emissions beyond the thresholds stated in the appropriate regulations.

The diagnosis shall be carried out by determining whether the measured oxygen sensor ceramic temperature falls below set limits over a number of measurement cycles. The evaluations of the diagnosis cycle are determined after the completion of a limited number of monitoring cycles.

Deviations in the oxygen sensor ceramic temperature or the oxygen sensor not being operatively ready in a timely manner (because of a too low temperature) can lead to an increase in emissions above the applicable standards or prevent the sensor signal from being used as a diagnostic system monitoring device. Deviations may occur due to, for example, ageing of the heater element, defective wiring, increased heater circuit connector contact resistance, defective heater driver etc.

1.14.2.1.1 General description

The purpose of this diagnosis is to detect electrical faults as defined in OBDI requirements. The input signal is a CAN message of instrument cluster. If an error is detected by the instrument cluster, the error symptom is sent via CAN to the ECU. The ECU then de-bounces the error and stores it in the error management.

Error Symptoms

1. short circuit to vbatt
2. short circuit to ground

1.14.2.2.1 General description

This diagnosis is performed in order to detect a stuck or not plausible AAT signal which cannot be detected by electrical range diagnosis.

The first part, just after start looks on the change of ambient temperature and compares the start and stop temperature. If the check is positive the diagnosis is finished. In negative case diagnosis runs to next step during warm up phase.

The error detection is only performed if the monitoring conditions for time after start, engine state idle speed, time of engine stop, ECT and ambient temperature are fulfilled. The plausibility error is detected if the absolute value of the temperature-difference between the arithmetic mean of engine coolant temperature ECT and temperature intake air IAT and the ambient temperature AAT (in formula: ABS (absolute value) of |(ECT+IAT) x 0,5 - AAT|) exceeds the threshold for an anti-bounce time.

The error validation is only performed if all electrical diagnoses for ECT and radiator outlet temperature are finished and the vehicle was driven with a certain vehicle speed. If both conditions are true and an error was detected, then the error is set for this driving cycle and the diagnosis is switched off.

For RBM handling the Cold Start Denominator will be considered.

Error Symptoms

1. ambient air temperature not plausible

1.14.3.1.1 General description

The purpose of this diagnosis is to detect electrical faults as defined in OBDI requirements. The input signal is analog from a NTC and has to be in a calibratible range. Short circuit to ground can be detected immediately, short circuit to voltage battery or open load after a delay time. If an error symptom is detected, the error counter is de-bounced.

Error Symptoms

1. Short circuit to voltage battery or open load
2. Short circuit to ground

1.14.3.2.1 General description

The purpose of this diagnosis is to detect an implausible jump discontinuity or implausible gradient or implausible offset on the intake air temperature signal. If a jump discontinuity is located, the error is not de-bounced and is registered in error management. If an implausible gradient or offset is detected, the error is de-bounced.

Error Symptoms

1. Signal gradient not plausible
2. Signal too high

1.14.3.3.1 General description

This diagnosis checks IAT integrity for a plausible range and signal stuck.

For the range detection, IAT has to be within coolant temperature and ambient temperature window. If IAT is outside of the range plus an offset, the error symptom is set and the error counter is de-bounced.

If the vehicle was driven with a certain vehicle speed for a calibratible time (IAT sensor cool down) and afterwards the vehicle was in idle for a calibratible time (IAT sensor hot up), the IAT signal must have moved. If the signal has not moved after a calibratible number of cool down/hot up phases, a stuck IAT signal is detected and the error is debounced.

For RBM handling the Cold Start Denominator will be considered.

Error Symptoms

1. Signal too high
2. Signal too low
3. Signal not plausible

1.14.4.1.1 General description

The ETC - H-Bridge IC continually checks the MTC if there is a short circuit to battery voltage or ground. In addition the IC is able to detect over temperature. This is performed internally to the ECU.

1.14.4.3.1 General description

This diagnosis is able to detect a too slow or jammed actuator. The given pulse width modulation signal (MTCPWM) exceeds the position controller permissible maximum value for longer than designated (Max short or Max Long) time.

If either of the times is exceeded, the appropriate DTC will be stored.

Also if a maximum allowed difference between throttle actual value and set-point value is exceeded, a DTC is stored.

1.14.8.1 General description

The purpose of the diagnosis is to detect faults of the knock sensor. Therefore the signal range and dynamics of a low pass filtered knock signal is checked.

If the signal range exceeds an upper or lower threshold a failure is detected.

An implausible knock signal is detected by using a statistical analysis. The difference between filtered knock signal and raw knock signal is estimated for a certain number of combustion cycles.

All error symptoms are de-bounced.

Error Symptoms

1. Noise level above valid range
2. Noise level below valid range
3. Knock sensor signal not plausible