2002 Toyota Tacoma System too Lean (Fuel Trim) Engine Diagnostics

Fuel trim is related to the feedback compensation value, not 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 compares 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 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.
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 ± 35 % (80°C (176°F) - or more), the system is functioning normally. - The A/F sensor (bank 1 sensor 1) output voltage and the short–term fuel trim value can be read using the hand–held tester or OBD II scan tool. - The ECM controls the voltage of the terminals AF1+ and AF1– of the ECM to the fixed voltage. Therefore it is impossible to confirm the A/F sensor output voltage without hand–held tester or OBD II scan tool. - OBD II scan tool (excluding hand–held tester) displays the one fifth of the A/F sensor (bank 1 sensor 1) output voltage which is displayed on the hand–held tester.

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