With A/T

NOTE: This article contains corrected data processed from TSB# PG001-05.

DTC P0171 System too Lean (Fuel Trim) (Only for A/T)(Bank 1)
DTC P0172 System too Rich (Fuel Trim) (Only for A/T)(Bank 1)
DTC P0174 System too Lean (Fuel Trim) (Only for A/T)(Bank 2)
DTC P0175 System too Rich (Fuel Trim) (Only for A/T)(Bank 2)

DTC Detecting Condition:

Wiring Diagram:

CIRCUIT DESCRIPTION
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 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.

HINT:
- When the DTC P0171 or P0174 is recorded, the actual air-fuel ratio is on the LEAN side. When DTC P0172 or P0175 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 or P0174 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, 2 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.
- The ECM controls the voltage of the terminals AFR+, AFL+, AFR- and AFL- 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 TOYOTA hand-held tester.
- OBD II scan tool (excluding TOYOTA hand-held tester) displays the one fifth of the A/F sensor output voltage which is displayed on the TOYOTA hand-held tester.

INSPECTION PROCEDURE

HINT: Read freeze frame data using TOYOTA hand-held tester or OBD II scan tool, as freeze frame data records the engine conditions when a 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.

Steps 1 - 2:

Steps 3 - 6:

Steps 7 - 8:

Steps 9 - 11: