Toyota Celica 2005, Repair Manual

Page: 645 / 2497

DI − 446
− DIAGNOSTICS ENGINE (2ZZ − GE)
2005 CELICA (RM1131U)
DTC P0171 System too Lean (Bank 1)
DTC P0172 System too Rich (Bank 1)
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 and carried out in long − term fueltrim to compensate
a continual deviation from the theoretical central value by the short − term fuel trim, due to individual engine’s
specifications and engine deterioration with time.
If both short
− term and long − term fuel trims are LEAN or RICH beyond a certain value, it is detected as a
malfunction and the MIL is illuminated.
DTC No. DTC Detection Condition Trouble Area
P0171
When air − fuel ratio feedback is stable after warming up the
engine, fuel trim is considered to be in error on LEAN side
(2 trip detection logic)
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 Open or short in heated oxygen sensor (bank 1, sensor 1)
circuit
S Heated oxygen sensor (bank 1, sensor 1)
S Heated oxygen sensor heater (bank 1, sensor 1)
S EFI relay
S PCV valve and hose
S PCV hose connection
S ECM
P0172
When air − fuel ratio feedback is stable after warming up the
engine, fuel trim is considered to be in error on RICH side
(2 trip detection logic)
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 Open or short in heated oxygen sensor (bank 1, sensor 1)
circuit
S Heated oxygen sensor (bank 1, sensor 1)
S Heated oxygen sensor heater (bank 1, sensor 1)
S EFI relay
S ECM
HINT:
S
When DTC P0171 is recorded, the actual air − fuel ratio is on the LEAN side. When DTC P0172 is re-
corded, the actual air − fuel ratio is on the RICH side.
S
If the vehicle runs out of fuel, the air − fuel ratio is LEAN and DTC P0171 may be recorded. The MIL
then comes on.
S
If the total of the short − term fuel trim value and long − term fuel trim value is within " 35 % (engine cool-
ant temperature is more than 75 _ C (167 _ F)), the system is functioning normally.
DIC7E − 02
644