| Table 1: | Fuel Trim Cells |
| Table 2: | DTC P0171 - Fuel Trim System Lean |
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 Bank 1 HO2S 1 and Bank 2 HO2S 1 signals 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 on Bank 1, the PCM will set DTC P0171. The PCMs maximum authority to control Long Term Fuel Trim allows a range between -20 percent and +20 percent. The PCMs maximum authority to control Short Term Fuel Trim allows a range between -20 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 are as follows:
Cell | Purge ON | Purge OFF |
|---|---|---|
Idle (Cell 0) | X | -- |
Decel (Cell 1) | -- | -- |
Normal (Cell 2) | X | X |
Accel (Cell 3) | X | X |
High Flow (Cell 4) | X | -- |
L.T. Fuel Trim average more than +17 percent | ||
S.T. Fuel Trim more than +2 percent | ||
The vehicle may have to be operated in all of the above conditions 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 VSS, TP Sensor, EVAP System, HO2S, Misfire, IAT Sensor, MAP Sensor, Idle Speed, Fuel Injectors, IC/Bypass, EGR Pintle, EGR Flow, ECT Sensor, CKP Sensor, CMP Sensor, or MAF Sensor DTCs set. |
| • | Engine coolant temperature is between 20°C-110°C (68°F-230°F). |
| • | Intake air temperature is between -18°C-70°C (0°F-158°F). |
| • | BARO is more than 70 kPa. |
| • | Manifold absolute pressure is between 15-85 kPa. |
| • | Throttle angle is less than 90 percent. |
| • | Vehicle speed is less than 82 mph. |
| • | Mass air flow is between 3-150 g/s. |
| • | Engine speed is between 600-4000 RPM. |
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. |
| • | Poor connection at the PCM -- Inspect harness connectors for the following conditions: |
| - | Backed out terminals |
| - | Improper mating |
| - | Broken locks |
| - | Improperly formed or damaged terminals |
| - | Poor terminal to wire connection |
| • | 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 Pressure Test (VIN K) or Fuel System Pressure Test (VIN 1) . |
| • | Fuel injectors 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. |
| • | 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. |
| • | 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 . |
| • | 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. |
Review the Failure 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.
-
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 Failure Records conditions are being duplicated, the lean condition is intermittent. Go 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? | -- | Go to applicable DTCs. Refer to Diagnostic Trouble Code (DTC) List/Type | ||
3 |
Are the displayed values more than the specified value? | +15% | ||
Does the scan tool indicate DTC P0171 failed this ignition? | -- | Go to Diagnostic Aids | ||
5 |
Did you find and correct the condition? | -- | ||
6 |
Did you find and correct the condition? | -- | ||
7 |
Did you find and correct the condition? | -- | ||
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 more than the specified value? | 5 | ||
10 |
Did you find and correct the condition? | -- | ||
11 |
Did you find and correct the condition? | -- | ||
12 | Check the fuel for excessive water, alcohol, or other contaminants. Refer to Alcohol/Contaminants-in-Fuel Diagnosis . Was a problem found? | -- | ||
13 |
Did you find and correct the condition? | -- | ||
14 |
Did you find and correct the condition? | -- | ||
15 |
Did you find and correct the condition? | -- | ||
16 |
Did you find and correct the condition? | -- | ||
17 |
Did you find and correct the condition? | -- | ||
18 |
Does S.T Fuel Trim decrease to near the specified value? | 0% | ||
19 |
Did you find and correct the condition? | -- | ||
20 |
Did you find and correct the condition? | -- | Go to Diagnostic Aids | |
21 | Inspect MAF sensor screen for foreign objects, debris, or damage. Was a problem found? | -- | ||
22 | Replace the MAF sensor. Refer to Mass Airflow Sensor Replacement . Is the action complete? | -- | -- | |
23 |
Does the scan tool indicate DTC P0171 failed this ignition? | -- | System OK |
