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 consists of the short-term fuel trim and the long-term fuel trim.
The short-term fuel trim is the short-term fuel compensation that is used to maintain the air-fuel ratio at stoichiometric air-fuel ratio. The signal from the Heated Oxygen (HO2) 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 injection volume if the air-fuel ratio is RICH, and an increase in the fuel injection volume if it is LEAN. Factors such as individual engine differences, wear overtime and changes in operating environment cause short-term fuel trim to vary from the central value. The long-term fuel trim, which controls overall fuel compensation, compensates for long-term deviations in the fuel trim from the central value caused by the short-term fuel trim compensation.
If both short-term and long-term fuel trims are LEAN or RICH beyond predetermined values, it is interpreted 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 Strategy:
Typical Enabling Conditions:
Typical Malfunction Thresholds:
Under closed-loop fuel control, fuel injection volumes that deviate from the ECM's estimated fuel injection volume causes changes 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 volume 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 malfunction 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 malfunction in the fuel system.
Wiring Diagram:
INSPECTION PROCEDURE
HINT:
Hand-held tester only:
Malfunctioning areas can be found by performing the ACTIVE TEST / A/F CONTROL operation (Heated Oxygen [HO2] sensor or other trouble areas). The A/F CONTROL operation can determine whether the front HO2 sensor (sensor 1), rear HO2 sensor (sensor 2) and other potential trouble areas are malfunctioning or not.
a. Perform A/F CONTROL using the hand-held tester.
HINT: The A/F Control operation lowers the fuel injection volume by 12.5 % or increases the injection volume by 25 %.
1. Connect the hand-held tester to the DLC3.
2. Turn the ignition switch to ON and turn the hand-held tester ON.
3. Warm up the engine by running the engine at 2,500 rpm for approximately 90 seconds.
4. On the hand-held tester, select the following menu items: DIAGNOSIS / ENHANCED OBD II / ACTIVE TEST / A/F CONTROL.
5. Perform the A/F CONTROL operation with the engine in an idling condition (press the right or left button).
Result:
Heated oxygen sensor reacts in accordance with increase and decrease of the fuel injection volume +25 % -> Rich output: More than 0.5 V (sensor 2), More than 0.55 V (sensor 1)
-12.5 % -> Lean output: Less than 0.4 V
NOTE: The front heated oxygen sensor has an output delay of a few seconds and the rear heated oxygen sensor has a maximum output delay of approximately 20 seconds.
Following A/F CONTROL procedure enables the technician to check and graph the voltage outputs of both the front and rear heated oxygen sensors.
To display the graph, enter ACTIVE TEST/A/F CONTROL / USER DATA, select "O2S B1S1 and O2S B1S2" by pressing the "YES" button followed by the "ENTER" button and then the "F4" button.
NOTE: If the vehicle is short of fuel, the air-fuel ratio becomes LEAN and heated oxygen sensor DTCs are set, and the MIL then comes on.
HINT:
- If different DTCs related to different systems that have terminal E2 as the ground terminal are output simultaneously, terminal E2 may have an open circuit.
- Read freeze frame data using the hand-held tester or the OBD II scan tool. Freeze frame data record the engine condition when malfunctions are detected. When troubleshooting, freeze frame data can help determine if the vehicle was moving or stationary, if the engine was warmed up or not, if the air-fuel ratio was lean or rich, and other data from the time the malfunction occurred.
- 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.
Step 1 - 3:
Step 4:
Step 5 - 6:
Step 7 - 10:
Step 11 - 15:
Step 16:
Hand-held tester:
Step 1 - 3:
Step 4:
Step 5 - 7:
Step 8 - 9:
Step 10 - 11:
Step 12:
Step 13 - 14:
Step 15 - 19:
Step 20 - 22:
OBD II scan tool (excluding the hand-held tester):
CHECK FOR INTERMITTENT PROBLEMS
HINT:
Hand-held tester only:
Inspect the vehicle's ECM using check mode. Intermittent problems are easier to detect with the hand-held tester when the ECM is in check mode. In check mode, the ECM uses 1trip detection logic, which is more sensitive to malfunctions than normal mode (default) that uses 2trip detection logic.
a. Clear DTCs.
b. Switch the ECM from normal mode to check mode using a hand-held tester.
c. Perform a simulation test.
d. Check and wiggle the harness(es), the connector(s) and terminal(s).