P0171
DTC P0171 SYSTEM TOO LEAN (BANK 1)CIRCUIT DESCRIPTION
DTC Detection Condition:
The fuel trim is related to the feedback compensation value, not to the basic injection time. The fuel trim includes the 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 stoichiometric air-fuel ratio. The signal from the heated oxygen sensor indicates whether the air-fuel ratio is RICH or LEAN compared to the stoichiometric air-fuel ratio. This variance triggers a reduction in the fuel volume if the air-fuel ratio is RICH, and an increase in the fuel volume if it is LEAN.
The long-term fuel trim is the overall fuel compensation carried out in long-term to compensate for a continual deviation of the short-term fuel trim from the central value, due to individual engine differences, wear overtime and changes in the operating 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 is illuminated.
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" may be 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% (engine coolant temperature is more than 75°C (167°F)), the system is functioning normally.
MONITOR DESCRIPTION
Monitor Description:
Monitor Strategy:
Typical Enabling Conditions:
Typical Malfunction Thresholds:
Under closed-loop fuel control, fuel injection amounts that deviate from the ECM's estimated fuel amount will cause a change in the long-term fuel trim compensation value. This long-term fuel trim is adjusted when there are persistent deviations in the short-term fuel trim values. And the deviation from a simulated fuel injection amount by the ECM affects a smoothed fuel trim learning value which is the combination of smoothed short term fuel trim (fuel feedback compensation value) and smoothed long term fuel trim (learning value of the air-fuel ratio). When the smoothed fuel trim learning value exceeds the DTC threshold, the ECM interprets this as a fault in the fuel system and sets a DTC.
Example:
The smoothed fuel trim leaning value is more than +35%, or less than -35%, the ECM interprets this as a fail in the fuel system.
Wiring Diagram:
Step 1 - 4:
Step 5 - 8:
Step 9 - 11:
Step (11 Continued- 13):
Step (13 Continued - 15):
Step 16 - 20:
Step 21 - 22:
INSPECTION PROCEDURE
HINT:
Hand-held tester only:
Narrowing down the trouble area is possible by performing the "A/F CONTROL" ACTIVE TEST (heated oxygen sensor or other trouble areas can be distinguished).
a. Perform ACTIVE TEST using the hand-held tester (A/F CONTROL).
HINT: "A/F CONTROL" is ACTIVE TEST which changes the injection volume -12.5% or +25%.
1. Connect the hand-held tester to the DLC3 on the vehicle.
2. Turn the ignition switch ON.
3. Warm up the engine by running the engine at 2,500 rpm for approximately 3 minutes.
4. Select the item "DIAGNOSIS/ENHANCED OBD II/ACTIVE TEST/ A/F CONTROL".
5. Perform "A/F CONTROL" with the engine in an idle condition (press the right or left button).
Result: Heated oxygen sensor reacts in accordance with increase and decrease of injection volume:
+25% -> rich output: More than 0.5 V
-12.5% -> lean output: Less than 0.4 V
NOTE: There is a delay of few seconds in the sensor 1 (front sensor) output, and there is about 20 seconds delay at maximum in the sensor 2 (rear sensor).
The following A/F CONTROL procedure enables the technician to check and graph the voltage outputs of both the heated oxygen sensors.
For displaying the graph indication, enter "ACTIVE TEST / A/F CONTROL / USER DATA", then select "O2S B1S1 and O2S B1S2" by pressing "YES" button and push "ENTER" button before pressing "F4" button.
HINT:
- If different DTCs related to different systems that have terminal E2 as the ground terminal are output simultaneously, terminal E2 may be open.
- Read freeze frame data using the hand-held tester or the OBD II scan tool. 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.
- A high heated oxygen sensor (sensor 1) voltage (0.5 V or more) could be caused by a rich air fuel mixture. Check for conditions that would cause the engine to run rich.
- A low heated oxygen sensor (sensor 1) voltage (0.4 V or less) could be caused by a lean air fuel mixture. Check for conditions that would cause the engine to run lean.
CHECK FOR INTERMITTENT PROBLEMS
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 the sensitivity to detect faults is increased. This makes it easier to detect intermittent problems.
a. Clear the DTCs.
b. Set the check mode.
c. Perform a simulation test.
d. Check the connector and terminal.
e. Wiggle the harness and connector.