Troubleshooting

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 oxygen sensor indicates whether the air-fuel ratio is RICH or LE1 compared to the ideal theoretical value, triggering a reduction in fuel volume if the air-fuel ratio is rich, ai 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 a 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:
- 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.
- If the vehicle runs out of fuel, the air-fuel ratio is LEAN and DTC P0171 is recorded. The MIL then comes on.
- If the total of the short-term fuel trim value and long-term fuel trim value is within ± 38 %, the system is functioning normally.
- The oxygen sensor output voltage and the short-term fuel trim value can be read using the OBD II scan tool or TOYOTA hand-held tester.

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INSPECTION PROCEDURE

HINT: Read freeze frame data using TOYOTA hand-held tester or OBD II scan tool. Because freeze frame records the engine conditions when the malfunction is detected, when troubleshooting it is useful for determining whether the vehicle was running or stopped, the engine warmed up or not, the air-fuel ratio lean or rich, etc. at the time of the malfunction.