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DI-52
DIAGNOSTICS - ENGINE (5VZ-FE)
DI5C7-08
DTC P0171 System too Lean (Fuel Trim)
DTC P0172 System too Lean (Fuel Trim)
CIRCUIT DESCRIPTION
Fuel trim refers to the feedback compensation value compared to 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 A/F sensor is approximately proportional to the existing air-fuel ratio,
and ECM comparing it with the ideal theoretical value, the ECM reduces fuel volume immediately if the air-
fuel ratio is rich and increases fuel volume if it is lean.
Long-term fuel trim compensates for the deviation from the central value of the short-term fuel trim stored
up by each engine tolerance, and the deviation from the central value due to the passage of time and
changes of environment.
If both the short-term fuel trim and long-term fuel trim exceed a certain value, it is detected as a malfunction
and the MIL lights up.
DTC No. DTC Detecting Condition Trouble Area
S Air induction system
S Injector blockage
S Mass air flow meter
S Engine coolant temp. sensor
When air fuel ratio feedback is stable after engine warming up,
S Fuel pressure
P0171 fuel trim is considerably in error on RICH side
S Gas leakage on exhaust system
(2 trip detection logic)
S Open or short in A/F sensor (bank 1 sensor 1) circuit
S A/F sensor (bank 1 sensor 1)
S PCV valve and hose
S ECM
S Injector leak, blockage
S Mass air flow meter
S Engine coolant temp. sensor
When air fuel ratio feedback is stable after engine warming up, S Ignition system
P0172 fuel trim is considerably in error on LEAN side S Fuel pressure
(2 trip detection logic) S Gas leakage on exhaust system
S Open or short in A/F sensor (bank 1 sensor 1) circuit
S A/F sensor (bank 1 sensor 1)
S ECM
HINT:
S When the 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.
S If the vehicle runs out of fuel, the air-fuel ratio is lean and DTC P0171 is 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 ◦ Jabse Service Manual Search 2026 ◦ Jabse Pravopis ◦ onTap.bg ◦ Other service manual resources online : Fixya ◦ eServiceinfo