Diagnostic Aids

WARNING: This page is about a different car, the 2007 Isuzu i-370, 2007 GMC Canyon, and 2007 Chevrolet Colorado. However, it is still accessible from the selected car via links, so may be relevant.

The current cell can be observed using a scan tool.
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 (w/Special Tool) or Alcohol/Contaminants-in-Fuel Diagnosis (w/o Special Tool) .
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 not pumping enough fuel, a plugged fuel filter or a restricted fuel line.
Insufficient fuel in the tank or low fuel pressure may cause this DTC to set. Refer to Fuel System Diagnosis .
Review the Failure Records with a scan tool. If an intermittent condition is suspected, refer to Intermittent Conditions .

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	IMPORTANT: Disregard any transmission symptoms, antilock brake system (ABS) indicators, and traction control system (TCS) indicators until any fuel trim faults are repaired. A fuel trim fault may cause default actions such as harsh shifts and illumination of the ABS/TCS indicators. Are any DTCs other than DTC P0171 also set?	-	Go to Diagnostic Trouble Code (DTC) List - Vehicle	Go to Step 3
3	Turn the ignition ON. Observe the Freeze Frame/Failure Records parameter with a scan tool and record the displayed data for this DTC. Start and operate the vehicle within the Conditions for Running the DTC. You may also operate the vehicle within the conditions observed from the Freeze Frame/Failure Records. Observe the Long Term FT parameter. Does the scan tool indicate that the Long Term fuel trim is more than the specified value?	23%	Go to Step 4	Go to Diagnostic Aids
4	Slowly operate the engine from idle to 3,000 RPM while observing the HO2S 1 and HO2S 2 parameters with a scan tool. Does the scan tool indicate that the heated oxygen sensor (HO2S) voltages 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: The vacuum hoses for splits, kinks, and proper connections-Refer to Emission Hose Routing Diagram . Fuel contamination-Refer to Alcohol/Contaminants-in-Fuel Diagnosis (w/Special Tool)Alcohol/Contaminants-in-Fuel Diagnosis (w/o Special Tool) . Did you find and correct the condition?	-	Go to Step 9	Go to Step 8
6	Turn OFF the engine. Inspect the HO2S for proper installation. Refer to Heated Oxygen Sensor 1 Replacement and Heated Oxygen Sensor 2 Replacement . 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 9	Go to Step 7
7	Test the HO2S high signal circuit for an open or short to ground. Refer to Circuit Testing and Wiring Repairs . 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, 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 Resonator and Outlet Duct Replacement . Evaporative canister purge lines plugged or obstructed Properly functioning fuel injectors-Refer to Fuel Injector Diagnosis (w/CH47976)Fuel Injector Diagnosis (w/J39021 or w/Tech 2) . Crankcase ventilation system for leaks-Refer to Crankcase Ventilation Hoses/Pipes Replacement . Did you find and correct the condition?	-	Go to Step 9	Go to Symptoms - Engine Mechanical
9	IMPORTANT: After repairs, use the scan tool Fuel Trim Reset function in order to reset the Long Term Fuel Trim. Turn ON the ignition, with the engine OFF. IMPORTANT: DO NOT clear codes with the engine running. The codes may reset in the same ignition cycle. 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 10
10	To verify that the performance of the catalytic converter has not been affected by the condition that set this DTC, perform the Repair Verification for DTC P0420. Refer to DTC P0420 . Did you complete the procedure?	-	Go to Step 11	Go to DTC P0420
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