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 fuel trim 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, the PCM will set DTC P0171.
The PCM's maximum authority to control long term fuel trim allows a range between -23 percent and +16 percent. The PCM's maximum authority to control short term fuel trim allows a range between -11 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 used to determine fuel trim are as follows:
| • | Idle, purge commanded (Cell 10). |
| • | Deceleration, purge commanded (Cell 1). |
| • | Normal driving, purge commanded (Cell 2). |
| • | Deceleration, no purge commanded (Cell 5). |
| • | Normal driving, no purge commanded (Cell 6). |
| • | Idle, no purge commanded (Cell 9). |
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 percent 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 active fuel trim, fuel injector circuit, Ignition Control circuit, Misfire, Idle speed, EVAP, HO2S, EGR, AIR, VSS, TP, IAT, MAP, ECT, CKP, CMP or MAF sensor DTCs present. |
| • | ECT between 20°C (68°F) and 110°C (230°F). |
| • | IAT is between -18°C (0°F) and 70°C (158°F). |
| • | BARO is greater than 70 kPa. |
| • | MAP is between 15 kPa and 85 kPa. |
| • | Throttle angle is less than 50 percent. |
| • | Vehicle speed is less than 83 mph. |
| • | MAF is between 3 g/s and 150 g/s. |
| • | Engine speed between 600 and 4000 RPM. |
Conditions for Setting the DTC
| • | Long term fuel trim is at or near maximum authority of +16 percent. |
| • | Short term fuel trim is at or near maximum authority of +20 percent. |
| • | All conditions have been met in Fuel Trim Cells 1, 2, 5, 6, 9 and/or 10. |
Action Taken When the DTC Sets
| • | The PCM illuminates the malfunction indicator lamp (MIL) on the second consecutive ignition cycle that the diagnostic runs and fails. |
| • | The PCM records the operating conditions at the time the diagnostic fails. The first time the diagnostic fails, the PCM stores this information in the Failure Records. If the diagnostic reports a failure on the second consecutive ignition cycle, the PCM records the operating conditions at the time of the failure. The PCM writes the conditions to the Freeze Frame and updates the Failure Records. |
Conditions for Clearing the MIL/DTC
| • | The PCM turns the MIL Off after three consecutive drive trips that the diagnostic runs and does not fail. |
| • | A last test failed (current DTC) clears when the diagnostic runs and does not fail. |
| • | A History DTC clears after forty consecutive warm-up cycles, if this or any other emission related diagnostic does not report any failures. |
| • | Use a scan tool in order to clear the MIL/DTC. |
| • | Interrupting the PCM battery voltage may or may not clear DTCs. This practice is not recommended. Refer to Powertrain Control Module (PCM) Description , Clearing Diagnostic Trouble Codes. |
Diagnostic Aids
Notice: Use the connector test adapter kit J 35616-A for any test that
requires probing the following items:
• The PCM harness connectors • The electrical center fuse/relay cavities • The component terminals • The component harness connector
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 Power 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, a DTC P0171 may be set. |
| Refer to Fuel System Pressure Test . |
| • | 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 Diagnosis . |
| - | 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 System Leak Diagnosis 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 . |
Reviewing the Failure Records vehicle mileage since the diagnostic test last failed may help determine how often the condition that caused the DTC to be set occurs. This may assist in diagnosing the condition.
If the problem is intermittent, refer to Intermittent Conditions .
Test Description
Number(s) below refer to the step number(s) 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 Fail Records conditions are being duplicated, the lean condition is intermittent. Refer to Diagnostic Aids for additional information on diagnosing intermittent conditions.
Step | Action | Value(s) | Yes | No |
|---|---|---|---|---|
1 | Did you perform the Powertrain On-Board Diagnostic (OBD) System Check? | -- | ||
Are any DTCs set other than P0171? | -- | Diagnose the other DTCs first. | ||
3 |
Are the displayed values more positive than the specified value? | +15% | ||
Does the scan tool indicate that this test ran and passed? | -- | Refer to Diagnostic Aids. | ||
5 | Visually and physically inspect the vacuum hoses for splits, kinks, and improper connections. Refer to Emission Hose Routing Diagram . Was a problem found and corrected? | -- | ||
6 | Visually and physically inspect the crankcase ventilation valve for proper installation and operation. Refer to Crankcase Ventilation System Diagnosis Was a problem found and corrected? | -- | ||
7 | Inspect the throttle body, duct work and the MAF sensor for damage or for the presence of foreign objects which may alter the airflow sample through the MAF sensor. Was a problem found and corrected? | -- | ||
8 | Start the engine and note the idle quality. Is a high or unsteady idle being experienced? | -- | ||
9 | With the engine idling, observe IAC Position on the scan tool. Is IAC Position greater than the specified value? (Value is displayed in counts). | 5 | ||
10 | Visually and physically inspect the throttle body, intake manifold, EGR valve and the EGR feed pipes for vacuum leaks. Was a problem found and corrected? | -- | ||
11 |
Was a problem found? | -- | ||
12 | Check the fuel for excessive water, alcohol, or other contaminants. Refer to Alcohol/Contaminants-in-Fuel Diagnosis . Was a problem found? | -- | ||
13 |
Was a problem found and corrected? | -- | ||
14 |
Does Short Term Fuel Trim decrease to near the specified value? | 0% | ||
15 | Perform the Evaporative Emissions Control (EVAP) System service bay test. Refer to DTC P0440 Evaporative Emission (EVAP) System . Was problem found and corrected? | -- | ||
16 | Perform the Fuel System Pressure Test. Refer to Fuel System Pressure Test . Was problem found and corrected? | -- | ||
17 | Visually and physically inspect the intake manifold, injector O-rings, EGR adapter, EGR valve and the EGR feed pipes for vacuum leaks. Was a problem found and corrected? | -- | ||
18 | Visually and physically inspect the exhaust manifold for leaks and loose or missing hardware. Was a problem found and corrected? | -- | ||
19 | Check fuel injector operation. Refer to Fuel Injector Balance Test . . Was a problem found and corrected? | -- | ||
20 |
Was a problem found and corrected? | -- | Refer to Diagnostic Aids | |
21 | Replace the MAF sensor. Refer to Mass Airflow Sensor Replacement . Is action complete? | -- | -- | |
22 |
Does the scan tool indicate that this test ran and passed? | -- | ||
23 | Review Captured Info using the scan tool. Are there any DTCs that have not been diagnosed? | -- | Go to the applicable DTC table | System OK |
