Theory of Operation
The fuel feedback system will maintain a stoiciometric fuel/air mixture, 14.7:1, by modifying the injector pulse width
according to the oxygen content of the exhaust gas. The PCM makes short term and long term fuel corrections to
maintain stoiciometric fuel/air ratio for best catalytic converter efficiency. Short term fuel correction is based on
upstream O2 sensor output and is designed for quick engine response. The long term fuel correction compensated
for variations in the engine specifications, sensor tolerances and component aging and is designed to correct rich
and lean conditions over a longer period of time.
•
When Monitored:
With the engine running in closed loop mode, the ambient/battery temperature above 20 deg. F and altitude
below 8500 ft.
•
Set Condition:
If the PCM multiplies short term compensation by long term adaptive and a purge fuel multiplier and the result
is below a certain value for 30 seconds over two trips, a freeze frame is stored, the MIL illuminates and a
trouble code is stored. Two Trip Fault. Three good trips to turn off the MIL.
Possible Causes
O2 SENSOR HEATER OPERATION
EVAP PURGE SOLENOID OPERATION
O2 SIGNAL CIRCUIT
O2 RETURN CIRCUIT
O2 SENSOR
MAP SENSOR
ECT SENSOR
ENGINE MECHANICAL PROBLEM
FUEL FILTER/PRESSURE REGULATOR (HIGH)
PCM
Always perform the Pre-Diagnostic Troubleshooting procedure before proceeding. (Refer to 9 - ENGINE -
DIAGNOSIS AND TESTING).
Diagnostic Test
1.
ACTIVE DTC
NOTE: Check for contaminates that may have damaged an O2 Sensor: contaminated fuel, unapproved sil-
icone, oil and coolant.
Ignition on, engine not running.
With a scan tool, read DTCs.
Is the DTC active at this time?
Yes
>> Go To 2
No
>> Refer to the INTERMITTENT CONDITION Diagnostic Procedure.
Perform the POWERTRAIN VERIFICATION TEST. (Refer to 9 - ENGINE - STANDARD PROCEDURE)
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ENGINE ELECTRICAL DIAGNOSTICS
KJ