Diagnostic Aids

The system will become lean if a fuel injector is not supplying enough fuel.
A lean condition could be present during high fuel demand due to a fuel pump that does not pump enough fuel, a plugged fuel filter, or a restricted fuel pipe.
Fuel contamination, such as water and alcohol, will affect the fuel trim.
Review the Failure Records with a scan tool. If an intermittent condition is suspected, refer to Intermittent Conditions .
Test 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 Alcohol/Contaminants-in-Fuel Diagnosis (without Special Tool) or Alcohol/Contaminants-in-Fuel Diagnosis (With Special Tool) .

DTC P0171

Step	Action	Values	Yes	No
1	Did you perform the Diagnostic System Check - Vehicle?	-	Go to Step 2	Go to Diagnostic System Check - Vehicle in Vehicle DTC Information
2	Are any DTCs other than P0171 also set?	-	Go to Diagnostic Trouble Code (DTC) List - Vehicle in Vehicle DTC Information	Go to Step 3
3	Install a scan tool. Review the Freeze Frame/Failure Records and record the displayed data for this DTC. Select Fuel and Emission data list. Start the engine. Observe the Long Term FT parameter with a scan tool. Is the Long Term FT parameter more than the specified value?	20%	Go to Step 4	Go to Diagnostic Aids
4	Operate the engine at idle. Observe the heated oxygen sensor (HO2S) parameters with a scan tool. Does the scan tool indicate that the HO2S parameters are within the specified range and fluctuating?	200-800 mV	Go to Step 5	Go to Step 6
5	Turn OFF the engine. Visually and physically inspect the following items: Splits, kinks, or improper connections at the vacuum hoses-Refer to Emission Hose Routing Diagram . Low fuel pressure-Refer to Fuel System Diagnosis . Properly functioning fuel injectors-Refer to Fuel Injector Coil Test . Fuel contamination-Refer to Alcohol/Contaminants-in-Fuel Diagnosis (without Special Tool) or Alcohol/Contaminants-in-Fuel Diagnosis (With Special Tool) . Did you find and correct the condition?	-	Go to Step 9	Go to Step 8
6	Turn OFF the engine. Turn ON the ignition, with the engine OFF. Observe the manifold absolute pressure (MAP) sensor pressure parameter with a scan tool. The MAP sensor pressure should be within the range specified for your altitude. Refer to Altitude vs Barometric Pressure . Do the MAP parameters indicate the correct barometric pressure?	-	Go to Step 7	Go to DTC P0106
7	Turn OFF the engine. Inspect for the following conditions: Proper HO2S installation Electrical connectors and wires are secured and not contacting the exhaust system. An open or a short to ground in the HO2S signal circuit-Refer to Circuit Testing and Wiring Repairs in Wiring Systems. Did you find and correct the condition?	-	Go to Step 9	Go to Fuel System Diagnosis
8	Operate the engine at idle. Inspect for the following conditions: Missing, loose, or leaking exhaust components Vacuum leaks at the intake manifold, throttle body, and injector O-rings Leaks at the air intake ducts Leaking crankcase ventilation system Did you find and correct the condition?	-	Go to Step 9	Go to Symptoms - Engine Mechanical in Engine Mechanical
9	IMPORTANT: After repairs, use the scan tool Fuel Trim Reset function in order to reset the Long Term Fuel Trim. Clear the DTCs with a scan tool. Turn OFF the ignition for 30 seconds. Start the engine. Operate the vehicle within the Conditions for Running the DTC. You may also operate the vehicle within the conditions that you observed from the Freeze Frame/Failure Records. Did the DTC fail this ignition?	-	Go to Step 2	Go to Step 10
10	Observe the Capture Info with a scan tool. Are there any DTCs that have not been diagnosed?	-	Go to Diagnostic Trouble Code (DTC) List - Vehicle in Vehicle DTC Information	System OK