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 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%; if the HO2S signals are indicating a lean condition the PCM will add fuel, resulting in fuel trim values above 0%. If a rich condition is detected, the fuel trim values will be below 0%, 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 -30% and +24%. The PCMs maximum authority to control short term fuel trim allows a range between -27% and +27%. 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) | -- | -- |
L.T. Fuel Trim average greater than +24% | ||
S.T. Fuel Trim greater than +27% | ||
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% 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 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 15 kPa and 85 kPa. |
| • | Throttle angle is steady and less than 90%. |
| • | Vehicle speed is less than 132 km/h (82 mph). |
| • | Engine speed is between 600 and 4000 RPM. |
| • | Baro is greater than 70 kPa. |
| • | Air flow is between 3 gm/s and 150 gm/s. |
Conditions for Setting the DTC
| • | Long term fuel trim is at or near maximum authority of 24%. |
| • | Short term fuel trim is greater than 2%. |
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 and Failure Records data. |
Conditions for Clearing the MIL/DTC
| • | The PCM will turn OFF the MIL during the third consecutive trip in which the diagnostic has been 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 the 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. |
| Power and Grounding Component Views. |
| • | Poor connection at the PCM - Inspect harness connectors for the following conditions: |
| - | Backed out terminals. Refer to Repairing Connector Terminals in Wiring Systems. |
| - | Improper mating. Refer to Repairing Connector Terminals in Wiring Systems. |
| - | Broken locks. Refer to Connector Repairs in Wiring Systems. |
| - | Improperly formed or damaged terminals. Refer to Repairing Connector Terminals in Wiring Systems. |
| - | Poor terminal to wire connection. Refer to Repairing Connector Terminals 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 injector(s) faulty - Refer to Fuel Injector Balance Test . |
| • | Vacuum leaks - Check 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 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. Check for exhaust leaks that may cause a false lean condition to be indicated. Refer to Exhaust System Inspection . |
| • | 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 PCM - Inspect harness connectors for backed out terminals, improper mating, broken locks, improperly formed or damaged terminals, and poor terminal to wire connection(s). Refer to Intermittents and Poor Connections Diagnosis , Repairing Connector Terminals and Connector Repairs in Wiring Systems. |
| • | Damaged harness - Inspect the wiring harness for damage. If the harness appears to be OK, observe the affected HO2S 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 fault. Refer to Wiring Repairs in Wiring Systems. |
Review the Fail Records vehicle mileage since the diagnostic test last failed. This may help determine how often the condition that caused the DTC to be set occurs.
Test Description
The numbers below refer to the step numbers on the Diagnostic Table:
Step | Action | Value(s) | Yes | No | ||||
|---|---|---|---|---|---|---|---|---|
1 | Was the Powertrain On Board Diagnostic (OBD) System Check performed? | -- | ||||||
Are any DTCs set other than P0171? | -- | Go to the applicable DTC charts and repair the other DTCs before proceeding with this chart | ||||||
3 |
Are the displayed values at or near the specified values?
| +16% +20% | ||||||
Does the scan tool indicate DTC P0171 failed this ignition? | -- | Go to Diagnostic Aids | ||||||
5 | Visually and physically inspect the vacuum hoses for splits, kinks, and improper connections and repair any problem found. Did your inspection reveal a problem requiring repair? | -- | ||||||
6 | Visually and physically inspect the crankcase ventilation valve for proper installation and repair any problem found. Refer to Crankcase Ventilation System Inspection . Did your inspection reveal a problem requiring repair? | -- | ||||||
7 |
Did your inspection of the throttle body reveal a condition requiring repair? | -- | ||||||
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 greater than the specified value? (Value is displayed in counts). | 5 | ||||||
10 |
Did your inspection reveal a vacuum leak? | -- | ||||||
11 | Perform Idle Air Control (IAC) Valve Check and correct any IAC problem as necessary. Did Idle Air Control System Check isolate a problem requiring repair? | -- | ||||||
12 | Check the fuel for excessive water, alcohol, or other contaminants. Refer to Alcohol/Contaminants-in-Fuel Diagnosis , and correct the contaminated fuel condition if present. Was the fuel contaminated? | -- | ||||||
13 |
Did your inspection reveal a condition requiring repair? | -- | ||||||
14 |
Does S.T. FUEL TRIM value decrease to near the specified value? | 0% | ||||||
15 | Perform the Fuel System Pressure Test and repair fuel system problem if necessary. Did the Fuel System Pressure Test isolate a condition requiring repair? | -- | ||||||
16 | Perform the Evaporative Emission Control System Diagnosis and repair EVAP system problem if necessary. Did the EVAP Control System Diagnosis isolate a problem? | -- | ||||||
17 |
Did your inspection reveal a problem? | -- | ||||||
18 | Visually and physically inspect the exhaust manifold for leaks and loose or missing hardware and correct any problem found. Did your inspection reveal a problem? | -- | ||||||
19 | Perform the Fuel Injector Balance Test , and correct any problem found. Did the Fuel Injector Balance Test isolate a problem? | -- | ||||||
20 |
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 repair? | -- | -- | |||||
22 |
Does the scan tool indicate DTC P0171 failed this ignition? | -- | Repair complete |
