Corolla LE Sedan L4-1.8L (1ZZ-FE) (1999)
Air/Fuel Mixture: Diagnostic Trouble Code Tests and Associated Procedures
DTC P0171 - System Too Lean (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, 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 form 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.
DETECTING CONDITIONS
HINT:
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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.
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If the vehicle runs out of fuel, the air-fuel ratio is LEAN and DTC P0171 is recorded. The MIL then comes on.
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If the total of the short-term fuel trim value and long-term fuel trim value is within ± 25 %, the system is functioning normally.
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The heated oxygen sensor (bank 1 sensor 1) output voltage and the short-term fuel trim value can be read using the OBD II scan tool or TOYOTA
hand-held tester.
Step 1