System Too Lean (Fuel Trim); System Too Rich (Fuel Trim) - Toyota T100 1996 Service Manual

DTC P0171
DTC P0172
CIRCUIT DESCRIPTION
Fuel trim refers to the feedback compensation value compared against the basic injection time. Fuel trim
includes short−term fuel trim and long−term fuel trim.
Short−term fuel trim is the short−term fuel compensation used to maintain the air−fuel ratio at its ideal
theoretical value. The signal from the heated oxygen sensor indicates whether the air−fuel ratio is RICH or
LEAN compared to the ideal theoretical value, triggering a reduction in fuel volume if the air−fuel ratio is rich,
and an increase in fuel volume if it is lean.
Long−term fuel trim is overall fuel compensation carried out long−term to compensate for continual deviation
of the short−term fuel trim from the central value due to individual engine differences, wear over time and
changes in the usage environment.
If both the short−term fuel trim and long−term fuel trim are LEAN or RICH beyond a certain value, it is de-
tected as a malfunction and the MIL lights up.
DTC No.
When air fuel ratio feedback is stable after engine warming
P0171 up, fuel trim is considerably in error on RICH side
(2 trip detection logic)
When air fuel ratio feedback is stable after engine warming
P0172 up, fuel trim is considerably in error on LEAN side
(2 trip detection logic)
HINT:
When the DTC P0171 is recorded, the actual air−fuel ratio is on the LEAN side. When DTC P0172 is
S
recorded, 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 is recorded. The MIL then
S
comes on.
If the total of the short−term fuel trim value and long−term fuel trim value is within ± 25 %, the system
S
is functioning normally.
INSPECTION PROCEDURE
1 Check air induction system (See page MF−1).
OK
1996 TOYOTA T100 (RM449U)
DIAGNOSTICS

System too Lean (Fuel Trim)

System too Rich (Fuel Trim)

DTC Detecting Condition
NG
− ENGINE (3RZ−FE)
S Air intake (hose loose)
S Fuel line pressure
S Injector blockage
S Heated oxygen sensor (bank 1 sensor 1) malfunction
S Mass air flow meter
S Engine coolant temp. sensor
S Fuel line pressure
S Injector leak, blockage
S Heated oxygen sensor (bank 1 sensor 1) malfunction
S Mass air flow meter
S Engine coolant temp. sensor
Repair or replace.
Trouble Area
DI−55
DI0TO−03