Circuit Description
To provide the best possible combination of driveability, fuel economy, and emission control, a closed loop air/fuel metering system is used. While in Closed Loop, the PCM monitors the HO2S 1 signal and adjusts fuel delivery based upon the HO2S signal voltage. A change made to fuel delivery will be indicated by the Long and Short Term FT values which can be monitored with a scan tool. Ideal Fuel Trim values are around 0 percent; if the HO2S signals are indicating a lean condition the PCM will add fuel, resulting in Fuel Trim values above 0 percent. If a rich condition is detected, the Fuel Trim values will be below 0 percent, indicating that the PCM is reducing the amount of fuel delivered. If an excessively lean condition is detected, DTC P0171 will set .
The PCMs maximum authority to control Fuel Trim allows a range between approximately -25 percent and +20 percent. The PCM monitors Fuel Trim under various operating conditions (fuel trim cells) before determining the status of the fuel trim diagnostic. The Fuel Trim cells are weighted (assigned a percentage of importance in determining total Fuel Trim status for the diagnostic) as follows:
| • | Idle, purge commanded (Cell 0 - approximately 10 percent) |
| • | Deceleration, purge commanded (Cell 1- approximately 15 percent). |
| • | Normal driving, purge commanded (Cell 2- approximately 25 percent) |
| • | Normal driving, no purge commanded (Cell 6- approximately 45 percent |
| • | Idle, no purge commanded (Cell 9- approximately 5 percent |
The vehicle may have to be operated in all of the above conditions before the PCM determines fuel trim status. The amount that fuel trim deviates from 0% in each cell and the amount that each cell is weighted directly affects whether or not the vehicle must be operated in all of the cells described above to complete the diagnostic.
Conditions for Running the DTC
| • | No VSS, TP sensor, EVAP system, HO2S, Misfire, IAT sensor, MAP sensor, idle speed, fuel injectors, IC/Bypass, EGR Pintle, EGR Flow, ECT sensor, CKP sensor, CMP sensor or MAF sensor DTCs set. |
| • | Engine coolant temperature between 20°C (68°F) and 110°C (230°F). |
| • | Intake air temperature is between -18°C (0°F) and 70°C (158°F). |
| • | BARO is more than 70 kPa. |
| • | Manifold absolute pressure is between 15 kPa and 85 kPa. |
| • | Throttle angle is less than 90 percent. |
| • | Vehicle speed is less than 82 mph. |
| • | Mass air flow is between 3 g/s and 150 g/s. |
| • | Engine speed between 600 and 4000 RPM. |
Conditions for Setting the DTC
| • | Long Term FT is at or near maximum authority of approximately +20 percent. |
| • | Short Term FT is at or near maximum authority of approximately +20 percent. |
| • | All conditions have been met in Fuel Trim Cells 1, 2, 6, 9 and/or 0. |
Action Taken When the DTC Sets
| • | The PCM will illuminate the malfunction indicator lamp (MIL) during the second consecutive trip in which the diagnostic test has been run and failed. |
| • | The PCM will store conditions which were present when the DTC set as Freeze Frame/Failure Records data. |
Conditions for Clearing the MIL/DTC
| • | The PCM will turn OFF the malfunction indicator lamp (MIL) during the third consecutive trip in which the diagnostic has run and passed. |
| • | The history DTC will clear after 40 consecutive warm-up cycles have occurred without a malfunction. |
| • | The DTC can be cleared by using a scan tool. |
Diagnostic Aids
Check for the following conditions:
| • | Heated oxygen sensor wiring -- The sensor pigtail may be routed incorrectly and contacting the exhaust system. |
| • | Poor PCM to engine block grounds -- Ensure that the PCM grounds are clean, tight, and in the proper locations. Refer to Ground Distribution Schematics in Wiring Systems. |
| • | Fuel pressure -- The system will go lean if pressure is too low. The PCM can compensate for some decrease. However, if fuel pressure is too low, DTC P0171 may set. Refer to Fuel System Pressure Test (VIN K) . |
| • | Vacuum leaks -- Check for the following conditions: |
| - | Disconnected or damaged vacuum hoses, refer to Emission Hose Routing Diagram . |
| - | Incorrectly installed or malfunctioning crankcase ventilation valve, refer to Crankcase Ventilation System Inspection . |
| - | Vacuum leaks at the throttle body, EGR valve, and intake manifold mounting surfaces. |
| • | Exhaust leaks -- An exhaust leak may cause outside air to be pulled into the exhaust gas stream past the HO2S, causing the system to appear lean. Check for exhaust leaks that may cause a false lean condition to be indicated. Refer to Exhaust Leakage in Engine Exhaust. |
| • | Mass air flow sensor -- Disconnect the MAF sensor and see if the lean condition is corrected. If so, replace MAF sensor. Refer to Mass Airflow Sensor Replacement . |
| • | Fuel contamination -- Water, even in small amounts, can be delivered to the fuel injectors. The water can cause a lean exhaust to be indicated. Excessive alcohol in the fuel can also cause this condition. Refer to Alcohol/Contaminants-in-Fuel Diagnosis . |
| • | Poor connection at the PCM -- Inspect harness connectors for the following conditions: |
| - | Backed out terminals |
| - | Improper mating |
| - | Broken locks |
| - | Improperly formed or damaged terminals |
| - | Poor terminal to wire connection |
| • | Damaged harness -- Inspect the wiring harness for damage. |
| • | If the harness appears to be OK, observe the HO2S 1 display on the scan tool while moving connectors and wiring harnesses related to the engine harness. A change in the display will indicate the location of the malfunction. |
If the DTC cannot be duplicated and is determined to be intermittent, reviewing the Failure Records can be useful in determining when the DTC was last set. Also refer to Testing for Intermittent Conditions and Poor Connections in Wiring Systems.
Test Description
The numbers below refer to the step numbers on the diagnostic table.
-
DTCs other than P0171 may indicate that a condition is present which may cause a lean condition. If this is the case, repairing the condition which caused the other DTC will most likely correct the DTC P0171.
-
If the DTC P0171 test passes while the Failure Records conditions are being duplicated, the lean condition is intermittent. Refer to Diagnostic Aids for additional information on diagnosing intermittent conditions.
Step | Action | Values | Yes | No |
|---|---|---|---|---|
1 | Did you perform the Powertrain On-Board Diagnostic (OBD) System Check? | -- | ||
Are any DTCs set other than P0171? | -- | Diagnose applicable DTCs. Go to Diagnostic Trouble Code (DTC) List/Type . | ||
3 |
Are the displayed values more than the specified value? | +15% | ||
Does the scan tool indicate DTC P0171 failed this ignition? | -- | Go to Diagnostic Aids | ||
5 |
Did you find and correct the condition? | -- | ||
6 |
Did you find and correct the condition? | -- | ||
7 |
Did you find and correct the condition? | -- | ||
8 | Start the engine and note the idle quality. Is a high or unsteady idle being experienced? | -- | ||
9 | With the engine idling, observe the idle air control display on the scan tool. Is the displayed value more than the specified value? | 5 counts | ||
10 |
Did you find and correct the condition? | -- | ||
11 |
Did you find and correct the condition? | -- | ||
12 | Test the fuel for excessive water, alcohol, or other contaminants. Refer to Alcohol/Contaminants-in-Fuel Diagnosis . Did you find and correct the condition? | -- | ||
13 |
Did you find and correct the condition? | -- | ||
14 |
Did you find and correct the condition? | -- | ||
15 |
Did you find and correct the condition? | -- | ||
16 |
Did you find and correct the condition? | -- | ||
17 |
Did you find and correct the condition? | -- | ||
18 |
Does Short Term FT decrease to near the specified value? | 0% | ||
19 |
Did you find and correct the condition? | -- | ||
20 |
Did you find and correct the condition? | -- | Go to Diagnostic Aids | |
21 | Inspect MAF Sensor screen for foreign objects, debris, or damage. Did you find and correct the condition? | -- | ||
22 | Replace the MAF sensor. Refer to Mass Airflow Sensor Replacement . Did you complete the replacement? | -- | -- | |
23 |
Does the DTC reset? | -- | System OK |
