Circuit Description
To provide the best possible combination of driveability, fuel economy, and emission control, a closed loop air and 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 in the fuel delivery will be indicated by changes in the long and short term fuel trim values. The fuel trim values can be monitored with a scan tool. Ideal fuel trim values are approximately 0 percent. If the HO2S signals indicate a lean condition, the PCM will add fuel, resulting in fuel trim values of more than 0 percent. If a rich condition is detected, the fuel trim values will be less than 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--or fuel trim cells--before determining the status of the fuel trim diagnostic. The fuel trim cells used to determine the 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
| • | There are no active DTCs of any of the following types present: |
| • | Fuel trim |
| • | Fuel injector circuit |
| • | Ignition control circuit |
| • | Misfire |
| • | Idle speed |
| • | Evaporative emission (EVAP) |
| • | Heated oxygen sensor (HO2S) |
| • | Exhaust gas recirculation (EGR) |
| • | Secondary air injection system (AIR) |
| • | Vehicle speed sensor (VSS) |
| • | Throttle position (TP) sensor |
| • | Intake air temperature (IAT) sensor |
| • | Manifold absolute pressure (MAP) sensor |
| • | Engine coolant temperature (ECT) sensor |
| • | Cranshaft postion (CKP) sensor |
| • | Camshaft position (CMP) sensor |
| • | Mass air flow (MAF) sensor |
| • | The ECT is between 20°-110°C (68°-230°F). |
| • | The IAT is between -18°C and +70°C (0°-158°F). |
| • | The BARO is more than 70 kPa (10.15 psi). |
| • | The MAP is between 15-85 kPa (2.175-12.327 psi). |
| • | The throttle angle is less than 50 percent. |
| • | The vehicle speed is less than 133.57 km/h (83.0 mph). |
| • | The MAF is between 3-150 g/s. |
| • | The engine speed is between 600-4000 RPM. |
Conditions for Setting the DTC
| • | The long term fuel trim is at or near maximum authority of +16 percent. |
| • | The short term fuel trim is at or near maximum authority of +20 percent. |
| • | All of the necessary 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 3 consecutive drive trips during which the diagnostic runs and passes. |
| • | A last test failed, or the current DTC, clears when the diagnostic runs and passes. |
| • | A History DTC clears after 40 consecutive warm-up cycles, if no other emission related diagnostic failures are reported. |
| • | Use a scan tool in order to clear the MIL diagnostic trouble code. |
| • | Interrupting the PCM battery voltage may or may not clear DTCs. This practice is not recommended. Refer to Powertrain Control Module 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 the following components and conditions:
| • | The heated oxygen sensor wiring -- The sensor pigtail may be routed incorrectly and contacting the exhaust system. |
| • | Faulty PCM to engine block grounds -- Ensure that the PCM grounds are clean, tight, and in the correct locations. Refer to Ground Distribution Schematics in Wiring Systems. |
| • | The fuel pressure -- The system will go lean if the pressure is too low. The PCM can compensate for some decrease. However, if the 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 . |
| - | An 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 be causing a false lean condition to be indicated. Refer to Exhaust System Inspection . |
| • | The mass air flow sensor -- Disconnect the MAF sensor and see if the lean condition is corrected. If so, replace the 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 occurs that caused the DTC to set. This may assist in diagnosing the condition.
If the problem is intermittent, refer to Intermittent Conditions .
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 Fail 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 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? | -- | Go 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 that 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 the idle high or unsteady? | -- | ||
9 | With the engine idling, observe IAC Position on the scan tool. Is the IAC Position more than the specified value?. | 5 counts | ||
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 the Short Term Fuel Trim decrease to near the specified value? | 0% | ||
15 | Perform the Fuel System Pressure Test. Refer to Fuel System Pressure Test . Was problem found and corrected? | -- | ||
16 | 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? | -- | ||
17 | Visually and physically inspect the exhaust manifold for leaks and loose or missing hardware. Was a problem found and corrected? | -- | ||
18 | Check fuel injector operation. Refer to Fuel Injector Balance Test with Tech 2 . Was a problem found and corrected? | -- | ||
19 |
Was a problem found and corrected? | -- | Go to Diagnostic Aids | |
20 | Replace the MAF sensor. Refer to Mass Airflow Sensor Replacement . Is action complete? | -- | -- | |
21 |
Does the scan tool indicate that this test ran and passed? | -- | ||
22 | Review the Captured Info using the scan tool. Are there any DTCs that have not been diagnosed? | -- | Go to the applicable DTC table | System OK |
