Troubleshooting
DTC 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 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 the fuel volume if the air-fuel ratio is rich, and an increase in the fuel volume if it is lean.
Long-term fuel trim is overall fuel compensation carried out in long-term to compensate for continual deviation of the short-term fuel trim form the central value, due to individual engine differences, wear over time and 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 (CHK ENG) lights up.
HINT: 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:
Step 1 - 4:
Step 5 - 7:
Step 8 - 9:
Step 10 - 13:
INSPECTION PROCEDURE
HINT: Read freeze frame data using LEXUS 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 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, the 1 trip detection logic is possible instead of the 2 trip detection logic and sensitivity to detect open circuits is increased. This makes it easier to detect intermittent problems.
1. Clear the DTC.
2. Set the check mode.
3. Perform a simulation test.
4. Check the connector and terminal.
5. Handle the connector.
CONFIRMATION DRIVING PATTERN
a. Connect the LEXUS hand-held tester to the DLC3.
b. Switch the LEXUS hand-held tester from Normal Mode to 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. Let the engine idle for 20 seconds or more.
f. Let the engine idle 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 followed, detection of the malfunction will impossible.
If you do not have a LEXUS hand-held tester, turn the ignition switch OFF after performing steps (c) to (f), then perform steps (c) to (f) again.