Troubleshooting

P0171 - System Too Lean (Fuel Trim, Bank 1)

CIRCUIT DESCRIPTION

DTC Detecting Condition:

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 heated oxygen sensor indicates whether the air-fuel ratio is RICH or LEAN compared to the ideal theoretical value, triggering a reduction in fuel volume if the air-fuel ratio is rich and 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 overtime arid changes in the using 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.

HINT:
- When 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 ±25 %, the system is functioning normally.

Wiring Diagram:

CONFIRMATION DRIVING PATTERN
a. Connect the LEXUS hand-held tester to the DLC3.
b. Switch the LEXUS hand-held tester from the normal mode to the check mode.
c. Start the engine and let the engine idle for 100 seconds. or more.
d. Drive the vehicle at 40 km/h (24 mph) or more for 20 seconds. or more.
e. Idle the engine for 20 seconds. or more.
f. Idle the engine for 30 seconds.

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

NOTE:
If the conditions in this test are not strictly satisfied, a malfunction cannot be detected.
If you do not have a LEXUS hand-held tester, turn the ignition switch OFF once after performing steps (c) to (f), then perform steps (c) to (f) again.

Step 1 - 5:

Step 6 - 8:

Step 9 - 12:

INSPECTION PROCEDURE

HINT: Read freeze frame data using LEXUS hand-held tester or OBD II scan tool. Because 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.

CHECK FOR INTERMITTENT PROBLEMS
LEXUS hand-held tester only:
By putting the vehicle's ECM in the check mode, 1 trip detection logic is possible instead of 2 trip detection logic and 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. Check the connector connection.