BMW 5251 Service Manual page 964

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On-Board Diagnostics
Professional diagnostic scan tools available atthe time of this
printing include the BMW factory tools (DISplus, GTI,
MoDiC) and a small number of aftermarket BMW-specific
tools. See 020 Maintenance.
In addition to the professional line of scan tools, inexpensive
generic OBD II scan tool software programs and handheld
units are readily available. Though limited, they are
nonetheless powerful diagnostic tools. These tools read live
data streams and freeze frame data as well as a host of other
valuable diagnostic data.
Diagnostic monitors
Diagnostic monitors run tests and checks on specific
emission control systems, components, and functions.
A complete drive cycle is requiredforthe tests to bevalid. See
Drive cycle in this repair group. The diagnostic monitor
signals the ECM of the loss or impairment of the signal or
component and determines if a signal or sensor is faulty
based on 3 conditions:
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Signal or component shorted to ground
Signal or component shorted to B+
Signal or component missing (open circuit)
The OBD II system monitors all emission control systems that
are installed. Emission control systems vary by vehicle model
and year. For example, a vehicle may not be equipped with
secondary air injection, so no secondary air readiness code
would be present.
OBD II software monitors the following:
Oxygen sensors
Catalysts
Engine misfire
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Fuel tank evaporative control system
Secondary air injection
Fuel system
Oxygen sensor monitoring. When driving conditions allow,
response rate and switching time of each oxygen sensor is
monitored. The oxygen sensor heater function is also
monitored. The OBD II system differentiates between
precataylst and post-catalyst oxygen sensors and reads each
one individually. In order forthe oxygen sensor to be
effectively monitored, the system must be in closed loop
operation.