Diagnose Functions - Overview - Volvo B6324S Functional Description

The Engine Management System fuel/ignition system
control module has an on-board diagnostic system
for self- diagnosis, continuously monitoring input and
output signals and several other functions.
If the engine control module (ECM) detects a fault,
some parameters will switch to predetermined, default
values, to allow as normal as possible operation. At
this time a pending diagnostic trouble code (DTC)
will be stored together with a number of relevant
parameters, to help the fault tracing operation. If the
same fault occurs during the next driving cycle then
the DTC will be set as permanent and if the fault is
emission relevant the malfunction indicator (MIL) will
be activated.
DIAGNOSE FUNCTIONS – OVERVIEW
If a fault disappears after the DTC has been stored,
information about the fault is stored in the ECM.
Every time the fault reoccurs a counter counts it. After
three consecutive driving cycles without the fault it is
allowed to turn off the MIL. For every warm-up cycle
that is driven without the fault reoccurring a second
counter counts down. It begins with 40 and counts
down to 0. When the second counter has counted
down to 0 the diagnostic trouble code can be erased
from the ECM memory.If the fault reoccurs the se-
cond counter is reset to 40.
The OnBoard Diagnostic (OBD) system also makes it
possible to read out the values and status of a number
of parameters through the diagnostic link connector
(DLC) using standardized protocol and a standardized
scan tool, or the manufacturers diagnostic tool.
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