Troubleshooting

P0171 - System Too Lean (Fuel Trim) (Bank 1)
CIRCUIT DESCRIPTION

DTC Detection Condition:

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 which is 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. 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 for the deviation from the central value of the short-term fuel trim which is 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 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 OBD II scan tool or TOYOTA hand-held tester.
- The ECM controls the voltage of the AF1+ and AF1- 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 TOYOTA hand-held tester.
- OBD II scan tool (excluding TOYOTA hand-held tester) displays the one fifth of the A/F sensor (bank 1 sensor 1) output voltage which is displayed on the TOYOTA hand-held tester.

Wiring Diagram:

CONFIRMATION DRIVING PATTERN

Confirmation Driving Pattern:

a. Connect the TOYOTA hand-held tester to the DLC3.
b. Switch the TOYOTA hand-held tester from the normal mode to the check mode.
c. Start the engine and warm it up with all the accessory switched OFF.
d. Drive the vehicle at 60 - 120 km/h (38 - 75 mph) and the engine speed at 1,600 - 3,200 rpm for 3 - 5 minutes.

HINT: If a malfunction exists, the MIL will light up during step (d).

NOTE:
- If the conditions in this test are not strictly followed, detection of the malfunction will not be possible.
- If you do not have a TOYOTA hand-held tester, turn the ignition switch OFF after performing steps (c) and (d), then perform steps (c) and (d) again.

Steps 1 - 4:

Step 5:

Steps 6 - 7:

Steps 8 - 11:

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

CHECK FOR INTERMITTENT PROBLEMS

TOYOTA hand-held tester only:
By pulling the vehicle's ECM in the check mode, 1 trip detection logic becomes possible instead of 2 trip detection logic and the sensitivity to detect open circuits is increased. This makes it easier to detect intermittent problems.

1. Clear the DTCs.
2. Set the check mode.
3. Perform a simulation test.
4. Check the connector and terminal.
5. Handle the connector.