Diagnostic Aids

The system will go lean if an injector is not providing enough fuel.
A lean condition could be present during high fuel demand due to a fuel pump that does not pump enough fuel.
Excessive resistance in the ignition 1 voltage and/or the fuel injector circuits may cause the following symptoms:
- A lean condition
- Misfire
- Rough idle
Fuel contamination, such as water and alcohol, will effect the fuel trim.
Review the Failure Records with a scan tool. If an intermittent condition is suspected, refer to Intermittent Conditions .
This DTC may set due to poor idle characteristics. A possible cause may be uncontrolled fueling due to an open or high resistance in the HO2S 1 low signal circuit. Before replacing any component, ensure that this condition does not exist.

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
2	Are any DTCs other than DTC P0171 also set?	-	Go to Diagnostic Trouble Code (DTC) List - Vehicle	Go to Step 3
3	Install the scan tool. Turn ON the ignition. Observe the Freeze Frame/Failure Records parameter with a scan tool and record the displayed data for this DTC. Start and idle the engine at normal operating temperature in the Closed Loop. Observe the Long Term FT parameter. Does the scan tool indicate that the Long Term fuel trim is more than the specified value?	18.8%	Go to Step 4	Go to Diagnostic Aids
4	Operate the engine at idle. Observe the heated oxygen sensor (HO2S) 1 sensor parameter with a scan tool. Does the scan tool indicate that the HO2S 1 voltage is 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: Vacuum hoses for splits, kinks, and proper connections - Refer to Emission Hose Routing Diagram . Ensure that the vehicle has sufficient fuel in the tank. If the fuel pressure is too low, this DTC may set. Refer to Fuel System Diagnosis . Fuel contamination - Refer to Alcohol/Contaminants-in-Fuel Diagnosis . Any lean fuel injectors - Refer to Fuel Injector Balance Test with Tech 2 . Did you find and correct the condition?	-	Go to Step 10	Go to Step 9
6	Turn OFF the engine. Inspect both HO2S for proper installation. Refer to Heated Oxygen Sensor Replacement - Position 1 and Heated Oxygen Sensor Replacement - Position 2 . Ensure that the electrical connectors and wires are secured and not contacting the exhaust system. Did you find and correct the condition?	-	Go to Step 10	Go to Step 7
7	Test the HO2S 1 low signal circuit for an open or high resistance. Refer to Circuit Testing and Wiring Repairs . Did you find and correct the condition?	-	Go to Step 10	Go to Step 8
8	Test for a short between the signal circuit and the low reference circuit or ground. Refer to Circuit Testing and Wiring Repairs . Did you find and correct the condition?	-	Go to Step 10	Go to Fuel System Diagnosis
9	Operate the engine at idle. Inspect for the following conditions: Missing, loose, or leaking exhaust components, from the HO2S forward - Refer to Symptoms - Engine Exhaust . Vacuum leaks at the intake manifold, the throttle body, and the injector O-rings Air induction system and the air intake ducts for leaks or for a missing air filter element - Refer to Air Cleaner Assembly Replacement and Air Cleaner Intake Duct Replacement . Evaporative canister purge lines plugged or obstructed Properly functioning fuel injectors - Refer to Fuel Injector Coil Test . Crankcase ventilation system for leaks - Refer to Crankcase Ventilation System Inspection/Diagnosis . Did you find and correct the condition?	-	Go to Step 10	Go to Symptoms - Engine Mechanical
10	IMPORTANT: Reset the Long Term Fuel Trim with the scan tool Fuel Trim Reset function after any of the above repairs. Turn ON the ignition, with the engine OFF. 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 in 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 11
11	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	System OK