Oxygen sensor pre-cat post cat BMW X5 2003 E53 M54 Engine Workshop Manual

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M54engMS43/ST036/6/2000
MS 43 CARRY OVER FUNCTIONS
INPUT FUNCTIONS
BOSCH OXYGEN SENSORS
The MS43.0 system uses Bosch LSH 25 oxygen sensors that function basically the same
as previously used (in Bosch systems). The voltage range is between 0 - 800 mV.
The location remains the same with the pre-cat sensors are mounted on top of the exhaust
manifolds. The catalysts are now integral with the exhaust manifolds (further detailed in the
M52 TU engine section).
PRE 02 SENSOR POST 02 SEN-
SOR
POST-CATALYST
SENSORS

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M54engMS43/ST036/6/20000
OXYGEN SENSOR SIGNAL INFLUENCE ON INJECTOR “OPEN” TIME
The ECM monitors the:
• Amplitude of the signal (highest voltage or range sensor is producing)
• Switching time of the signal (how fast from lean to rich)
• Frequency of complete cycles (how many within a period of time)
These characteristics provide info to the ECM that reflect the overall condition of the sen-
sor.
POST CATALYTIC CONVERTER SENSOR SIGNAL
The post catalyst O2 sensors monitor the efficiency of the catalyst as a requirement of OBD
II. This signal also provides feedback of the pre-catalyst sensors efficiency and can cause
the ECM to “trim” the ms injection time to correct for slight deviations.
• If the catalyst is operating efficiently, most of the remaining oxygen in the exhaust gas
is burned (lack of O2 - “constant lean signal”).
The sensor signal fluctuates slightly in the higher end of the voltage scale.
• If the post sensor shows excessive fluctuations
(which echo the scope pattern of the pre
sensor), this indicates that the catalytic converter is not functioning correctly and cannot
consume the O2 (fault set).
• If the post sensor fluctuations move out of the normal voltage “window”, this indicates
that the pre sensor is not performing properly due toslight
deterioration. These systems
can also “trim” the ms injection time to compensate for this.
The constantly changing oxygen sensor input to the ECM is needed to correct the ms
injection time to ensure that the ideal air/fuel ratio is maintained.