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 powertrain control module (PCM) monitors the HO2S 1 signal and adjusts fuel delivery based upon the HO2S signal voltages. 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 PCMs maximum authority to control long term fuel trim allows a range between -29-+22 percent. The PCMs maximum authority to control short term fuel trim allows a range between -27 to +27 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 as follows:
Cell | Purge ON | Purge OFF |
|---|---|---|
Idle (Cell 0) | X | -- |
Decel (Cell 1) | X | -- |
Normal (Cell 2) | X | -- |
Accel (Cell 3) | -- | -- |
High Flow (Cell 4) | -- | -- |
Long Term F.T. average more than +21% | ||
Short Term F.T. more than +2% | ||
The vehicle may have to be operated in all of the above conditions marked by an X, before the PCM determines fuel trim status. The amount of fuel trim deviates from 0 percent in each cell and the amount that each cell is used 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 MAF, MAP, TP, ECT, IAT, CKP, CAM Sensor, Misfire, Fuel Injector, VSS, EST, HO2S, EGR, or EVAP DTCs are set. |
| • | Engine coolant temperature is between 20°C (68°F) and 110°C (230°F). |
| • | Intake air temperature is between -18°C (0°F) and 70°C (158°F). |
| • | Manifold absolute pressure is between 18 kPa and 80 kPa. |
| • | Throttle angle is steady and less than 90 percent. |
| • | Vehicle speed is less than 132 km/h (82 mph). |
| • | Engine speed is between 600 and 4000 RPM. |
| • | Baro is more than 70 kPa. |
| • | Air flow is between 3 gm/s and 150 gm/s. |
Conditions for Setting the DTC
| • | Long term fuel trim average is more than 21 percent. |
| • | Short term fuel trim is more than 2 percent. |
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
Inspect 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-- Refer to Ground Distribution Schematics in Wiring Systems. |
| • | Fuel pressure low--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 Diagnosis . |
| • | Fuel injectors faulty--Refer to Fuel Injector Balance Test . |
| • | Vacuum leaks--Inspect for disconnected or damaged vacuum hoses and for vacuum leaks at the intake manifold, throttle body, EGR system, and crankcase ventilation system. Refer to Visual/Physical Inspection in Symptoms . |
| • | 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. Inspect for exhaust leaks that may cause a false lean condition to be indicated. Refer to Exhaust System Inspection in Exhaust Systems. |
| • | 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 . |
Test Description
The numbers below refer to the step numbers on the diagnostic table:
Step | Action | Values | Yes | No | ||||
|---|---|---|---|---|---|---|---|---|
1 | Did you perform the Powertrain On Board Diagnostic (OBD) System Check? | -- | ||||||
Are any DTCs set other than P0171? | -- | Go to applicable DTCs | ||||||
3 |
Are the displayed values at or near the specified values?
| +21% +2% | ||||||
Does the scan tool indicate DTC P0171 failed this ignition? | -- | Go to Diagnostic Aids | ||||||
5 |
Did your inspection reveal a problem requiring repair? | -- | ||||||
6 |
Did the inspection reveal a problem requiring repair? | -- | ||||||
7 |
Did your inspection of the throttle body reveal a condition requiring repair? | -- | ||||||
8 |
Is a high or unsteady idle being experienced? | -- | ||||||
9 | With the engine idling, observe the IAC display on the scan tool. Is the displayed value more than the specified value? | 5 | ||||||
10 |
Did your inspection reveal a vacuum leak? | -- | ||||||
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 . Was the fuel contaminated? | -- | ||||||
13 |
Did your inspection reveal a condition requiring repair? | -- | ||||||
14 |
Does Short Term F.T. value decrease to near the specified value? | 0% | ||||||
15 | Perform the procedure in Fuel System Pressure Test and repair fuel system problem if necessary. Refer to Fuel System Pressure Test . Did the Fuel System Pressure Test isolate a condition requiring repair? | -- | ||||||
16 | Perform the EVAP Control System Diagnosis and repair EVAP system problem if necessary. Refer to Evaporative Emission Control System Diagnosis . Did you find and correct the condition? | -- | ||||||
17 |
Did your inspection reveal a problem? | -- | ||||||
18 |
Did your inspection reveal a problem? | -- | ||||||
19 | Perform the Injector Balance Test and correct any problem found. Refer to Fuel Injector Balance Test . Did the Injector Balance Test isolate a problem? | -- | ||||||
20 | Visually and physically inspect the HO2S to ensure that it is installed securely and that the HO2S pigtail and wiring harness are not contacting the exhaust or otherwise damaged. Did your inspection reveal a problem? | -- | Go to Diagnostic Aids | |||||
21 | Replace the MAF sensor. Refer to Mass Airflow Sensor Replacement . Did you complete the replacement? | -- | -- | |||||
22 |
Does the scan tool indicate DTC P0171 failed this ignition? | -- | System OK |
