Troubleshooting
DTC P0171 System too Lean (Bank 1)CIRCUIT DESCRIPTION
DTC Detection Condition:
The fuel trim refers to the feedback compensation value compared to the basic injection time. The fuel trim includes short-term fuel trim and the long-term fuel trim.
The 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 it it is lean.
The 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 and changes in the usage environment.
If both the short-term fuel trim and the 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 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 the 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 ±38 %, the system is functioning normally.
- The heated oxygen 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 the TOYOTA hand-held tester.
Wiring Diagram:
Step 1 - 3:
Step 4 - 7:
Step 8 - 9:
Step 10 -12:
INSPECTION PROCEDURE
HINT: Read freeze frame data using the hand-held tester or the OBD II scan tool, as freeze frame data records the engine conditions when a malfunction is detected. When troubleshooting, this 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
TOYOTA 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 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 TOYOTA hand-held tester to the DLC3.
b. Switch the TOYOTA hand-held tester from normal mode to check mode.
c. Start the engine and let the engine idle for 100 sec. or more.
d. Drive the vehicle at 40 km/h (24 mph) or more for 20 sec. or more.
e. Let the engine idle for 20 sec. or more.
f. Let the engine idle for 30 sec.
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 not be possible. If you do not have a TOYOTA hand-held tester, turn the ignition switch OFF after performing steps (c) to (f), then perform steps (c) to (f) again.