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 FTvalues 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 PCM's maximum authority to control Long Term FT allows a range between -23% and +16%. The PCM's maximum authority to control short term fuel trim allows a range between -11% and +20%. 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 0). |
| • | Deceleration, purge commanded (Cell 1). |
| • | Normal driving, purge commanded (Cell 2). |
| • | Deceleration, no purge commanded (Cell 6). |
| • | Normal driving, no purge commanded (Cell 7). |
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 Setting the DTC
| • | No MAF sensor, ECT sensor, HO2S, TP sensor, EVAP, EGR, Misfire, Injector circuit, Idle speed, CKP sensor, CMP Senosor, Vehicle speed, EST, MAP sensor, or IAT 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 greater than 70 kPa. |
| • | Manifold absolute pressure is between 18 kPa and 80 kPa. |
| • | Throttle angle is less than 90%. |
| • | Vehicle speed is less than 82 mph. |
| • | Mass air flow is between 3 gm/s and 150 gm/s. |
| • | Engine speed between 600 and 4000 rpm. |
| • | Long Term FT is at or near maximum authority of +16%. |
| • | Short Term FT is at or near maximum authority of +20%. |
| • | All conditions have been met in Fuel Trim Cells. |
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. |
| • | 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 . |
| • | Fuel injector(s). 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 Emission Hose Routing Diagram . |
| • | 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. |
| • | MAF 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 PCM. Inspect harness connectors for backed out terminals, improper mating, broken locks, improperly formed or damaged terminals, and 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 fault. |
Reviewing the Fail 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.
Test Description
The number(s) below refer to the step number(s) on the Diagnostic Table:
-
DTCs other than P0171 may indicate a condition 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 | Was the Powertrain OBD System Check performed? | -- | ||||||
Are any DTCs set other than P0171? | -- | Refer to applicable DTC Table | ||||||
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 |
Was a problem found? | -- | ||||||
6 |
Was a problem found? | -- | ||||||
7 |
Was a problem found? | -- | ||||||
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 |
Was a problem found? | -- | ||||||
11 |
Was a problem found? | -- | ||||||
12 |
Was the fuel contaminated? | -- | ||||||
13 |
Was a problem found? | -- | ||||||
14 |
Decrease to near the specified value? | 0% | ||||||
15 |
Was problem found? | -- | ||||||
16 |
Was problem found? | -- | ||||||
17 |
Was a problem found? | -- | ||||||
18 |
Was a problem found? | -- | ||||||
19 |
Was a problem found? | -- | ||||||
20 |
Was a problem found? | -- | Go to Diagnostic Aids | |||||
21 | Replace the MAF sensor. Refer to Mass Airflow Sensor Replacement . Is the action complete? | -- | -- | |||||
22 |
Does the scan tool indicate DTC P0171 failed this ignition? | -- | System OK |
