Toyota Celica 2005 Repair Manual page 347

DI−148
DTC P0171
DTC P0172
CIRCUIT DESCRIPTION
The fuel trim is related to the feedback compensation value, not to the basic injection time. The fuel trim in-
cludes the short−term fuel trim and the long−term fuel trim.
The short−term fuel trim is the short−term fuel compensation used to maintain the air−fuel ratio at stoichio-
metric air−fuel ratio. The signal from the heated oxygen sensor indicates whether the air−fuel ratio is RICH
or LEAN compared to the stoichiometric air−fuel ratio. This variance triggers a reduction in the fuel volume
if the air−fuel ratio is RICH, and an increase in the fuel volume if it is LEAN.
The long−term fuel trim is the overall fuel compensation carried out in long−term fuel trim to compensate for
a continual deviation of the short−term fuel trim from the central value, due to individual engine differences,
wear overtime and changes in the operating environment.
If both the short−term fuel trim and the long−term fuel trim are LEAN or RICH beyond a certain value, it is
detected as a malfunction and the MIL is illuminated.
DTC No.
When air−fuel ratio feedback is stable after warming up engine,
P0171 fuel trim is considerably in error on LEAN side
(2 trip detection logic)
When air−fuel ratio feedback is stable after warming up engine,
P0172 fuel trim is considerably in error on RICH side
(2 trip detection logic)
HINT:
When DTC P0171 is recorded, the actual air−fuel ratio is on the LEAN side. When DTC P0172 is re-
S
corded, the actual air−fuel ratio is on the RICH side.
If the vehicle runs out of fuel, the air−fuel ratio is LEAN and DTC P0171 may be recorded. The MIL
S
then comes on.
If the total of the short−term fuel trim value and long−term fuel trim value is within " 35 % (engine cool-
S
ant temperature is more than 75 _C (167_F)), the system is functioning normally.
2005 CELICA (RM1131U)
DIAGNOSTICS
System too Lean (Bank 1)
System too Rich (Bank 1)
DTC Detection Condition
− ENGINE (1ZZ−FE)
S Air induction system
S Injector blockage
S Mass air flow meter
S Engine coolant temperature sensor
S Fuel pressure
S Gas leakage in exhaust system
S Heated oxygen sensor (bank 1, sensor 1)
S EFI relay
S PCV valve and hose
S ECM
S Injector leak, blockage
S Mass air flow meter
S Engine coolant temperature sensor
S Ignition system
S Fuel pressure
S Gas leakage in exhaust system
S Heated oxygen sensor (bank 1, sensor 1)
S EFI relay
S ECM
346
Trouble Area
DIC6S−02