Toyota 1 MZ–FE Manual page 486

EG2–486
DTC P0171 System too Lean (Fuel Trim)
DTC P0172 System too Rich (Fuel Trim)
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, triggening 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
detected as a malfunction and the MIL lights up.
Diagnostic Trouble Code Detecting Condition
DTC No.
When the air fuel ratio feedback is stable after
engine warming up, the fuel trim is considerably in
P0171
error on the RICH side.
(2 trip detection logic)
When the air fuel ratio feedback is stable after
engine warming up, the fuel trim is considerably in
P0172
error on the LEAN 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
recorded, the actual air–fuel ratio is on the RICH side,
Fuel trim applies separately to bank 1 and bank 2, so the ECM lights up the MIL if a problem occurs with
either bank.
You can tell which bank is malfunctioning by looking at the short–term fuel trim and long–term fuel trim,
thus allowing you to focus your inspection.
If the total of the short–term fuel trim value and long–term fuel trim value for each bank is within
25%, the system is functioning normally.
1MZ–FE ENGINE – CIRCUIT INSPECTION
Air intake (hose loose)
Fuel line pressure
Injector blockage
Heated oxygen sensor malfunction
Mass air flow meter
Engine coolant temp. sensor
Fuel line pressure
Injector leak, blockage
Heated oxygen sensor malfunction
Mass air flow meter
Engine coolant temp. sensor
Trouble Area