Troubleshooting

DTC P0171 SYSTEM TOO LEAN (FUEL TRIM) (BANK 1)
DTC P0172 SYSTEM TOO RICH (FUEL TRIM) (BANK 1)


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 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 using 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.




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 ± 35 % (80 °C (176 °F) or more), the system the system is functioning normally.
- The A/F sensor (bank 1, 2 sensor 1) output voltage and the short-term fuel trim value can be read using the OBD It scan tool or LEXUS hand-held tester.
- The ECM controls the voltage of the AF+ and AF- terminals of the ECM to the fixed voltage. Therefore, it is impossible to confirm the A/F sensor output voltage without OBD II scan tool or LEXUS hand-held tester.
- OBD II scan tool (excluding LEXUS hand-held tester) displays the one fifth of the A/F sensor output voltage which is displayed on the LEXUS hand-held tester.




WIRING DIAGRAM


































INSPECTION PROCEDURE

HINT: Read freeze frame data using LEXUS 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 was warmed up or not, the air-fuel ratio was lean or rich, etc. at the time of the malfunction.