Saab 9000 - Page 143

140 Fault diagnosis
Diagnostic trouble code P0300
Misfiring, randomly in several cylinders.
Symptom of fault
The CHECK ENGINE lamp (MIL) is on or flashing.
Uneven running when misfiring.
Possibly other diagnostic trouble codes.
Conditions
An engine load and speed dependent matrix in the
electronic control module's memory specifies how
large a proportion of misfires is required for exhaust
emissions to reach prohibited limits or for the three
way catalytic converter to be damaged.
If the number of misfires exceeds the matrix value,
a diagnostic trouble code will be generated. Misfiring
occurs in several cylinders.
Diagnostic help
The cause of misfiring could be:
• Low fuel level in the tank. If this is the case it will be
Indicated by diagnostic trouble code P1416 also
being generated.
• a fault in the fuel system
- low fuel pressure
- purge valve open continuously
- incorrect voltage from front heated oxygen sen-
sor
- defective injectors
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• a fault in the ignition system
- defective or incorrect spark plugs
- defective ignition discharge module
• a fault in the basic engine
. - poor compression
• faults in common components
- pressure sensor
- main relay
- coolant temperature sensor.
The cause of incorrectly detected misfiring, diagnos-
tic trouble code P0300 without misfiring, could be:
• control module pin 17 or 18 shorted to ground
Momentary misfiring can be ascertained using an
ISAT scan tool.
- Select "READ FUNCTIONS".
- Select "MISFIRING CYLINDER 1".
Other cylinders can also be selected.
Saab 9000