Description
The DTC P0171 Fuel Trim System Lean diagnostic monitors the heated oxygen sensor indications of the air/fuel mixture. In order to provide the best possible combination of driveability, fuel economy, and emission control, the powertrain control module (PCM) uses a Closed Loop air/fuel metering system. The PCM monitors the heated oxygen sensor (HO2S) signal voltage and when in Closed Loop adjusts fuel delivery based on the HO2S signal voltage. Changes in fuel delivery will be indicated by the long term and the short term fuel trim values that are displayed on the scan tool. The ideal fuel trim value is around 0 percent. The PCM will add fuel when the heated oxygen sensor signal is indicating a lean condition. Additional fuel is indicated by fuel trim values that are above 0 percent. The PCM will reduce the amount of fuel delivered when a rich condition is indicated by the HO2S. Fuel trim values below 0 percent indicate a reduction in fuel. A fuel trim DTC can be set when exhaust emissions reach excessive levels because of a lean or rich condition.
HO2S Indication | Short Term FT Action | Long Term FT Action | Short Term FT Response |
|---|---|---|---|
Lean Condition | Quick Increase of 1% or More | Slow Increase of 1% or More | Return to 0% |
Desired Condition | Stays Near 0% | Stays at Learned +/- Value | Stays Near 0% |
Rich Condition | Quick Decrease of -1% or Less | Slow Decrease of -1% or Less | Return to 0% |
Conditions for Running the DTC
| • | The barometric pressure (BARO) is more than 75 kPa. |
| • | The engine coolant temperature (ECT) was -8 to +95°C (18-203°F) at engine start up. |
| • | The intake air temperature (IAT) is -8 to +60°C (18-140°F) at engine start up. |
Conditions for Setting the DTC
Any of the following conditions are detected while the ECT is less than 110°C (230°F):
| • | The short term fuel trim is more than or equal to 15 percent in 128 firing events. |
| • | The long term fuel trim is more than or equal to 20 percent in 128 firing events. |
| • | The total fuel trim is more than or equal to 43 percent in 128 firing events. |
Action Taken When the DTC Sets
| • | The PCM illuminates the malfunction indicator lamp (MIL) after 2 consecutive ignition cycles in which the diagnostic runs with the fault active. |
| • | The PCM records the operating conditions at the time the diagnostic fails. This information is stored in the Freeze Frame buffer. |
Conditions for Clearing the MIL/DTC
| • | The MIL turns off after 3 consecutively passing cycles without a fault present. |
| • | A history DTC will clear after 40 consecutive warm-up cycles without a fault. |
| • | Perform the scan tool Clear DTC Information function in order to clear the DTC. |
Diagnostic Aids
Check for any of the following conditions:
| • | Check for vacuum leaks. |
| • | Check for exhaust leaks. |
| • | Check for the correct fuel pressure. The fuel system will be lean if the fuel pressure is too low. In order to determine the cause of DTC P0171 monitoring the fuel pressure while driving the vehicle at various road speeds may be necessary. Refer to Fuel System Diagnosis . |
| • | Check for fuel contamination. Small amounts of water can be delivered to the fuel injectors and cause a lean exhaust indication. A lean exhaust indication can also be caused by too much alcohol in the fuel. Refer to Evaporative Emission Control System Diagnosis . |
| • | A malfunctioning MAF sensor |
| • | Check for a damaged wiring 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 the connectors and the wiring harnesses related to the sensor. A change in the display will indicate the location of the fault. |
| • | Check for a contaminated oxygen sensor. Silicone and other contaminants can cause the oxygen sensor to provide false lean exhaust readings. If contamination is found, determine the cause and correct the condition before replacing the HO2S. |
| • | Inspect the HO2S 1 for water intrusion into the wiring harness and the sensor housing. Water can create a short to ground in the HO2S signal circuit causing a false lean indication. |
| • | Check the HO2S 1 and the MAF sensor for a faulty electrical connection to the PCM. |
If a DTC P0171 is intermittent, driving the vehicle under the following conditions can verify whether the fault is present. Perform the scan tool Clear DTC Information function. Road test the vehicle while monitoring the DTC P0171 diagnostic on the scan tool under the Not Ran Since Code Cleared selection in the DTC Information menu. If a DTC P0171 appears in the Not Ran Since Code Cleared list, the P0171 diagnostic has not yet run. When the DTC P0171 does not appear in the Not Ran Since Code Cleared list, the P0171 diagnostic has run. If the MIL is NOT illuminated, and there is no Pending DTC Status in DTC Information, the P0171 diagnostic has passed. The DTCs MUST be cleared in order to view the Current Status of the Not Ran Since Code Cleared list. DO NOT forget that the Not Ran Since Code Cleared list only indicates that the test has run, not whether the test passed or failed. The DTC Information screen must be checked for Current or Pending status, in order to determine the outcome of the diagnostic test involved.
If a DTC P0171 cannot be duplicated, the information included in the Freeze Frame data can be useful in determining vehicle operating conditions when the DTC was first set.
Test Description
The numbers below refer to the step numbers in the diagnostic table.
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The Diagnostic System Check-Engine Controls prompts the technician to complete some basic checks and store the freeze frame data on the scan tool if applicable. This creates an electronic copy of the data taken when the fault occurred. The information is then stored in the scan tool for later reference.
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This step determines that the fault is present.
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Engine control sensors that are found to be out range or fixed, can cause the engine to run lean. Refer to the applicable sensor DTC table for direction in diagnosing any possible sensor error.
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This step checks for inadequate fuel delivery from a fuel injector. The fault may be a fuel injector or the fuel injector electrical circuit.
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Clearing the DTCs by removing the DOME fuse also clears the PCM long term memory. Using the scan tool to clear DTCs leaves the long term memory active. DTC emitting diagnostics may not run to completion as expected unless the long term memory is cleared. Removal of the DOME fuse also clears the time display of the radio. The DTC P0171 diagnostic can be monitored on the scan tool under the Not Ran Since Code Cleared selection in the DTC Information menu. If a DTC P0171 appears in the Not Ran Since Code Cleared list, the DTC P0171 diagnostic has not yet run. When the DTC P0171 does not appear in the Not Ran Since Code Cleared list, the DTC P0171 diagnostic has run. If the MIL is NOT illuminated, and there is no Pending DTC Status in DTC Information, the DTC P0171 diagnostic has passed. The DTCs MUST be cleared in order to view the Current Status of the Not Ran Since Code Cleared list. DO NOT forget that the Not Ran Since Code Cleared list only indicates that the test has run, not whether the test passed or failed. The DTC Information screen must be checked for Current or Pending status, in order to determine the outcome of the diagnostic test involved.
Step | Action | Value(s) | Yes | No | ||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|
Did you perform the Diagnostic System Check-Engine Controls? | -- | Go to Step 2 | ||||||||||
2 |
Is the scan tool display less than the specified value? | 15% | Go to Step 3 | Go to Step 4 | ||||||||
Was the scan tool display less than the specified value while operating the vehicle within the Freeze Frame conditions? | 43% | Go to Diagnostic Aids | Go to Step 4 | |||||||||
4 |
Did any of the above checks isolate a condition requiring a repair? | -- | Go to Step 11 | Go to Step 5 | ||||||||
Was a repair necessary? | -- | Go to Step 11 | Go to Step 6 | |||||||||
6 |
Is the fuel pressure within the specified range? | 250-300 kPa (36-43 psi) | Go to Step 7 | Go to Fuel System Diagnosis | ||||||||
7 | Check the fuel for excessive water, alcohol, or other contaminants. Refer to Alcohol/Contaminants-in-Fuel Diagnosis . Was the fuel contaminated? | -- | Go to Step 8 | Go to Step 9 | ||||||||
8 | Replace any contaminated fuel. Refer to Fuel System Cleaning . Is the action complete? | -- | Go to Step 11 | -- | ||||||||
Check the fuel injectors for any of the following conditions:
Did the fuel injectors pass all the checks? | -- | Go to Diagnostic Aids | Go to Step 10 | |||||||||
10 | Replace any faulty fuel injectors. Refer to Fuel Injector Replacement . Is the action complete? | -- | Go to Step 11 | -- | ||||||||
Does the DTC run and pass? | -- | Go to Step 12 | Go to Step 2 | |||||||||
12 | With a scan tool, observe the stored information, Capture Info. Does the scan tool display any DTCs that you have not diagnosed? | -- | System OK |
