When you say your scanner says both your post-cat HO2S's are fine, it simply means that they are recognized and operating. That doesn't mean they're operating correctly.
Your LTFT's tell you exactly why this code triggered;
Quote:
The average long term fuel trim value is above 19 percent.
The above condition is present for 90 seconds.
From GM;
Quote:
DTC P0171 or P0174
Circuit Description
The powertrain control module (PCM) controls the air/fuel metering system in order to provide the best possible combination of driveability, fuel economy and emission control. Fuel delivery is controlled differently during Open and Closed Loop. During Open Loop the PCM determines fuel delivery based on sensor signals without oxygen sensor input. During Closed Loop the PCM adds oxygen sensor inputs and level of purge to calculate Short and Long Term fuel trim adjustments. If the oxygen sensors indicate a lean condition, fuel trim values will be above 0 percent . If the oxygen sensors indicate a rich condition, fuel trim values will be below 0 percent. The values for the Short Term fuel trim change rapidly in response to the heated oxygen sensor (HO2S) voltage signals. Long Term fuel trim makes coarse adjustments in order to maintain an Air/Fuel Ratio of 14.7:1. A block of cells contain information arranged in combinations of engine RPM and engine load for a full range of vehicle operating conditions. The long term fuel trim diagnostic is based on an average of cells currently being used. The PCM selects the cells based on the engine speed and engine load. If the PCM detects an excessively lean condition, DTC P0171 or P0174 sets.
DTC Descriptors
This diagnostic procedure supports the following DTCs:
• DTC P0171 Fuel Trim System Lean Bank 1
• DTC P0174 Fuel Trim System Lean Bank 2
Conditions for Running the DTC
• DTCs P0030, P0036, P0050, P0056, P0101, P0102, P0103, P0107, P0108, P0121, P0122, P0123, P0130, P0131, P0132, P0133, P0134, P0135, P0137, P0138, P0140, P0141, P0142, P0150-P0158, P0160, P0161, P0201, P0202, P0203, P0204, P0205, P0206, P0207, P0208, P0300, P0401, P0403, P0404, P0405, P0410, P0442, P0443, P0446, P0449, P0452, P0453, P0455, P0496, P0506, P0507, P1133, P1134, P1153, P1154, P1404, are not set.
• The engine coolant temperature (ECT) is between -40 to +127°C (-40 to +261°F).
• The intake air temperature (IAT) is more than -7°C (+19.4°F).
• The manifold absolute pressure (MAP) is more than 5 kPa (0.73 psi).
• The vehicle speed is less than 134 km/h (83 mph).
• The engine speed is between 400-6,000 RPM.
• The barometric pressure (BARO) is more than 74 kPa (10.7 psi).
• The mass air flow (MAF) is more than 0.5 g/s.
• The fuel level is more than 10 percent.
• DTC P0171 and P0174 run continuously once the above conditions have been met.
• The engine is in Closed Loop fueling.
Conditions for Setting the DTC
The average long term fuel trim value is above 19 percent.
The above condition is present for 90 seconds.
Action Taken When the DTC Sets
• The control module illuminates the malfunction indicator lamp (MIL) on the second consecutive ignition cycle that the diagnostic runs and fails.
• The control module records the operating conditions at the time the diagnostic fails. The first time the diagnostic fails, the control module stores this information in the Failure Records. If the diagnostic reports a failure on the second consecutive ignition cycle, the control module records the operating conditions at the time of the failure. The control module writes the operating conditions to the Freeze Frame and updates the Failure Records.
Conditions for Clearing the MIL/DTC
• The control module turns OFF the malfunction indicator lamp (MIL) after 3 consecutive ignition cycles that the diagnostic runs and does not fail.
• A current DTC, Last Test Failed, clears when the diagnostic runs and passes.
• A history DTC clears after 40 consecutive warm-up cycles, if no failures are reported by this or any other emission related diagnostic.
• Clear the MIL and the DTC with a scan tool.
Diagnostic Aids
• The system will go lean if an injector is not suppling enough fuel.
• The system will go lean during high fuel demand.
• Using a scan tool, review the Failure Records. If an intermittent condition is suspected, refer to Intermittent Conditions .
• Excessive resistance in the fuel injector ignition 1 voltage circuit or the fuel injector control circuit may cause the following symptoms:
- Lean condition
- Misfire
- Rough idle
Test Description
The numbers below refer to the step numbers on the diagnostic table.
If no problems are found, a worn cam, worn intake or exhaust values or other engine mechanical failures may be at fault.
Step 1
Did you perform the Diagnostic System Check - Vehicle?
--
Yes: Go to Step 2
No: Go to Diagnostic System Check - Vehicle in Vehicle DTC Information
Step 2
Important: If any DTCs are set, other than P0171, refer to those DTCs before continuing.
- Install a scan tool.
- Start and idle the engine at normal operating temperature in Closed Loop.
- Record the Long Term Fuel Trim data.
- Turn OFF the engine.
- Turn ON the ignition, with the engine OFF.
Review the Freeze Frame/Failure Records and record displayed data for this DTC.
Does the scan tool indicate that the long term fuel trim is greater than the specified value?
19%
Yes: Go to Step 3
No: Go to Diagnostic Aids
Step 3
- Operate engine at idle.
- Observe the HO2S parameters with a scan tool.
Are the HO2S parameters within the specified range and fluctuating?
200-800 mV
Yes: Go to Step 4
No: Go to Step 5
Step 4
- Turn OFF the engine.
- Inspect the following conditions:
• Vacuum hoses for splits, restrictions, and improper connections--Refer to Emission Hose Routing Diagram .
• Ensure that the vehicle has sufficient fuel in tank. If fuel pressure is too low this DTC may set. Refer to Fuel System Diagnosis .
• Fuel contamination--Refer to Alcohol/Contaminants-in-Fuel Diagnosis .
Did you find and correct the condition?
--
Yes: Go to Step 7
No: Go to Step 6
Step 5
- Turn OFF the engine.
- Inspect for the following conditions:
• The heated oxygen sensor (HO2S) for proper installation
• The electrical connectors and the wires are secured and not contacting the exhaust system
Inspect for a short between the signal circuit and the low reference circuit.
Did you find and correct the condition?
--
Yes: Go to Step 4
No: Go to Fuel System Diagnosis
Step 6
Operate the engine at idle and inspect for the following conditions:
• Any missing, loose, or leaking exhaust components
• Any vacuum leaks at the intake manifold and injector O-rings
• The air induction system and air intake ducts for vacuum leaks and restrictions
• The crankcase ventilation system for leaks--Refer to Crankcase Ventilation System Inspection/Diagnosis in Engine Mechanical.
Did you find and correct the condition?
--
Yes: Go to Step 7
No: Go to Symptoms - Engine Mechanical in Engine Mechanical
Step 7
Important: After repairs, use the scan tool Fuel Trim Reset function in order to reset the Long Term Fuel.
-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 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?
--
Yes: Go to Step 2
No: Go to Step 8
Step 8
Observe the Capture Info with a scan tool.
Are there any DTCs that have not been diagnosed?
--
Yes: Go to Diagnostic Trouble Code (DTC) List - Vehicle in Vehicle DTC Information
No: System OK
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