Troubleshooting

DTC P0171 SYSTEM TOO LEAN (FUEL TRIM) (BANK 1)

CIRCUIT DESCRIPTION

DTC Detecting Condition:

Fuel trim refers to the feedback compensation value compared against 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 over time and changes in the usage 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 check engine warning (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.

CONFIRMATION DRIVING PATTERN

Confirmation Driving Pattern:

a. Switch the hand-held tester from the normal mode to the check mode. *1
b. Start the engine and let the engine idle for 100 seconds or more. *2
c. Drive the vehicle at 40 km/h (24 mph) or more for 20 seconds or more. *3
d. Let the engine idle for 20 seconds or more. *4
e. Repeat the procedure *3 and *4 three times.
f. Let the engine idle for 30 seconds. *5
g. Check the O2 sensor output voltage.

HINT: If a malfunction exists, the check engine warning light comes on.

NOTE: If the conditions on this test are not strictly followed, the malfunction will not be detected.

Wiring Diagram:

Step 1 - 6:

Step 7 - 9:

Step 10 - 13:

INSPECTION PROCEDURE

HINT: Read freed frame data using hand-held tester. Because freeze frame records the engine conditions when the 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

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.