Autodata - Diagnostic Trouble Codes Fault locations and probable causes - 2004 edition

How to use this manual
Fault location
Fuel trim --
FI~. 5
Short and long term fuel trim refers to the strategy
used to reduce exhaust emissions after the basic
computation of injection period, using engine load as
the major parameter.
Both front and rear oxygen sensor signals are used
to fine tune the fuellair mixture by increasing or
decreasing the injection period +I-25% above or
below the basic level.
Any fault requiring a correction beyond this level will
result in a trouble code being logged.
When the engine is new and running satisfactorily
a level of fuel trim will be established - represented
Fig. 5
-
by 100°/o.
The fuel trim will oscillate the injection period +I-5%
above and below the mean level [A].
Manufacturing and in-service tolerances of load
sensors (MAP, MAF or VAF) and injectors in
particular and faults such as intake air leaks will
affect the fuellair mixture and cause the fuel trim to
quickly compensate.
An intake air leak for instance would result in the
injection period increasing, for example to 115-125%. This level will also oscillate +I-5% as before [B].
This new short term fuel trim (STFT) level will be stored in the ECM if it is established as a new basic mixture level. It will
then become a long term fuel trim (LTFT) correction and results in the correct mixture level immediately after starting,
even when the H02S has had insufficient time to heat up.
Long term fuel trim (LTFT) values can be erased by disconnecting the ECM power supply for a suitable length of time.
If the memory is not erased after repairs the ECM will eventually learn the new LTFT values, but this will take some time
and probably cause high emissions and some driveability problems.
For example if an intake air leak causes the engine to run lean this will be compensated by a change in the LTFT value,
which will be stored in the ECM memory.
After the leak has been repaired this memorised LTFT value will still be used to compute the injection period, resulting in
excessively rich running, until new LTFT values have been learned.
RICHER
Probable causes
Wiring
Wiring - refers not only to the wiring harness, but also to any associated component multi-plugs, relay plates, welded,
crimped or soldered joints and bulkhead connectors.
Short to earth - refers to wiring that has a 'leak' to earth somewhere along its length e.g. if the insulation has chaffed
through and the wiring is in contact with the body or engine.
Short to positive - refers to wiring that would normally not be carrying 12 volts, but is shorted to the 12 volt supply e.g. a
sensor wire that would normally be fed with approx. 5 volts from the ECM, shorted to a 12 volt battery positive supply.
Open circuit - refers to wiring that is either disconnected or broken, but is not shorted to either earth or a positive supply.
Poor connection - refers to a loose, intermittent or high resistance connection.
General faults
Despite the sophistication of modern electronic management systems, basic mechanical faults can still cause unsatisfactory
starting, running and driveability.
Before assuming that a problem is an electronic one, it should be established that the engine/transmission/immobilizer is in
good mechanical condition and that the basic fuel and electrical circuits are OK.