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Nonstop CEL's after numerous "fixes"

mn15
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mn15 New Member

mn15

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2015 Sierra SLE 5.3L w/ 104k miles. I have been getting nonstop CEL codes for the past few months.  After replacing all 4 O2 sensors, purge valve evap, PCV, thermostat and cat, all codes go away after I clear them only to reappear when I drive it next.  Took it into a shop and they can find no vacuum leaks.  Current codes are P0172 bank 1 too rich, P0175 bank 2 too rich, P0430 catalyst system efficiency below threshold bank 2.  Truck drives fine most of the but rpms dip and at times it feels like there is a misfire, plugs were replaced at about 80k.  Any help would be greatly appreciated.  Thanks.

asilverblazer Senior Enthusiast

asilverblazer

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Diagnostic Instructions

DTC Descriptors

DTC P0171

Fuel Trim System Lean Bank 1

DTC P0172

Fuel Trim System Rich Bank 1

DTC P0174

Fuel Trim System Lean Bank 2

DTC P0175

Fuel Trim System Rich Bank 2

Circuit/System Description

The engine control module (ECM) controls a Closed Loop air/fuel metering system in order to provide the best possible combination of driveability, fuel economy, and emission control. The ECM monitors the heated oxygen sensor (HO2S) signal voltage, and adjusts the fuel delivery based on the signal voltage while in Closed Loop. A change made to the fuel delivery changes the long and short term fuel trim values. The short term fuel trim values change rapidly in response to the HO2S signal voltages. These changes fine tune the engine fueling. The long term fuel trim values change in response to trends in the short term fuel trim The long term fuel trim makes coarse adjustments to fueling in order to re-center and restore control to short term fuel trim. The ideal fuel trim values are around 0 %. A positive fuel trim value indicates that the ECM is adding fuel in order to compensate for a lean condition. A negative fuel trim value indicates that the ECM is reducing the amount of fuel in order to compensate for a rich condition.

Conditions for Running the DTC

  • DTCs P0030, P0050, P0053, P0059, P0101, P0102, P0103, P0106, P0107, P0108, P0131–P0135, P0151–P0155, P015A, P015B, P015C, P015D, P0178, P0179, P0201–P0208, P0261, P0262, P0264, P0265, P0267, P0268, P0270, P0271, P0273, P0274, P0276, P0277, P0279, P0280, P0282, P0283, P0300–P0308, P0442, P0455, P0458, P0496, P0506, P0507, P1248, P1249, P124A, P124B, P124C, P124D, P124E, P124F, P2147, P2148, P2150, P2151, P2153, P2154, P2156, P2157, P216B, P216C, P216E, P216F, P217B, P217C, P217E, P217F, P2227, P2228, P2229, P2230, or P2269 are not set.
  • The engine is in Closed Loop status.
  • The catalyst monitor diagnostic intrusive test, post HO2S diagnostic intrusive test, device control, and EVAP leak test are not active.
  • The engine coolant temperature is between −40 and +150°C (−40 and +302°F).
  • The intake air temperature is between −20 and +150°C (−4 and +302°F).
  • The manifold absolute pressure is between 10–255 kPa (1.5–37 psi).
  • The engine speed is between 375–7,000 RPM.
  • The mass air flow is between 1–510 g/s.
  • The barometric pressure is greater than 70 kPa (10.2 psi).
  • The fuel level is greater than 10 %.
  • This diagnostic runs continuously when the above conditions have been met.

Conditions for Setting the DTC

  • The average long term fuel trim weighted average value is greater or less than a calibrated value.
  • The above condition is present for approximately 3 min after the Conditions for Running the DTC have been met.

Action Taken When the DTC Sets

DTCs P0171, P0172, P0174, and P0175 are Type B DTCs.

Conditions for Clearing the MIL/DTC

DTCs P0171, P0172, P0174, and P0175 are Type B DTCs.

Diagnostic Aids

  • Allow the engine to reach operating temperature. With the engine running, observe the HO2S parameter with a scan tool. The HO2S value should vary from approximately 40 mV to approximately 900 mV, and respond to fueling changes.
  • The normal Short Term Fuel Trim and Long Term Fuel Trim parameters should be between +10 and −10 % with 0 % the optimum, with the engine running at operating temperature.
  • Any un-metered air into the engine causes this DTC to set. Thoroughly inspect all areas of the engine for vacuum leaks.
  • A MAF sensor condition can cause this DTC without setting a MAF DTC. If there is a MAF sensor condition, the MAF sensor parameters will appear to be within range.
  • Inspect the air filter for being the correct one for this application. Make sure that the engine oil fill cap is in place and that it is tight. Verify that the engine oil dip stick is fully seated.
  • Certain aftermarket air filters may cause a DTC to set.
  • Certain aftermarket air induction systems or modifications to the air induction system may cause a DTC to set.
  • Certain aftermarket exhaust system components may cause a DTC to set.

Reference Information

Schematic Reference

Engine Controls Schematics

Connector End View Reference

 

Component Connector End Views

Component View Reference

 

Powertrain Component Views

Electrical Information Reference

DTC Type Reference

 

Powertrain Diagnostic Trouble Code (DTC) Type Definitions

Scan Tool Reference

 

Control Module References for scan tool information

Circuit/System Verification

  1. Ignition ON.
  2. Verify no other DTCs are set.  
    • If no other DTCs are set

  3. Verify DTC P0171, P0172, P0174, or P0175 is not set.
    • If a DTC is set
      Refer to Circuit/System Testing.
     
    • If no DTC is set

  4. Operate the vehicle within the Conditions for Running the DTC. You may also operate the vehicle within the conditions that you observed in the Freeze Frame/Failure Records data.
  5. Verify DTC P0171, P0172, P0174, or P0175 is not set.
    • If any of the DTCs set
      Refer to Circuit/System Testing.
     
    • If none of the DTCs set

  6. All OK.

Circuit/System Testing

  1. Ignition ON.
  2. Verify the MAP sensor pressure parameter is within the range specified for your altitude. Refer to Altitude Versus Barometric Pressure.
    • If not within the specified range
      Refer to DTC P0106.
     
    • If within the specified range

  3. Engine running at the operating temperature.
  4. Verify the Long Term Fuel Trim Bank 1 or 2 parameter is between −10 % and +10 %.
    • If greater than +10 %:
     
    1. Ignition OFF.
    2. Inspect for the conditions listed below:
      • Mass air flow sensor signal skewed—If the Short Term Fuel Trim parameters for both banks changes greater than 20 % when the MAF sensor is disconnected, refer to DTC P0101.
      • Contamination of the mass air flow sensor with oil or other foreign material.
      • Fuel contamination—Refer to Alcohol/Contaminants-in-Fuel Diagnosis.
      • Missing, restricted, or leaking exhaust components—Refer to Symptoms - Engine Exhaust.
      • Malfunctioning fuel injectors—Refer to Fuel Injector Diagnosis.
      • Vacuum hoses for splits, kinks, and improper connections.
      • Insufficient fuel in the tank.
      • Partial blockage of the air filter element or other conditions that alter air flow to the mass air flow sensor.
      • Low fuel pressure—Refer to Fuel System Diagnosis.
      • Vacuum leaks at the intake manifold, the throttle body, and the injector O-rings.
      • Leaks in the air induction system and the air intake ducts.
      • Missing air filter element.
      • Cracked EVAP canister.
      • Evaporative pipes obstructed or leaking.
      • The crankcase ventilation system for leaks—Refer to Crankcase Ventilation System Inspection/Diagnosis.
        • If a condition is found, repair as necessary.
        • If no condition is found, inspect the engine for a mechanical condition. Refer to Symptoms - Engine Mechanical.


      •  
     
    • If less than −10 %
     
    1. Ignition OFF.
    2. Inspect for the conditions listed below:
      • Mass air flow (MAF) sensor signal skewed—If the Short Term Fuel Trim parameters for both banks changes greater than 20 % when the MAF sensor is disconnected, refer to DTC P0101.
      • Contamination of the mass air flow sensor with oil or other foreign material.
      • Fuel contamination—Refer to Alcohol/Contaminants-in-Fuel Diagnosis.
      • Missing, restricted, or leaking exhaust components—Refer to Symptoms - Engine Exhaust.
      • Malfunctioning fuel injectors—Refer to Fuel Injector Diagnosis.
      • Collapsed or restricted air intake duct.
      • Excessive fuel pressure — Refer to Fuel System Diagnosis.
      • Dirty or restricted air filter.
      • Objects blocking the throttle body.
      • Improper operation of the evaporative emissions control system.
      • Excessive fuel in the crankcase—Change the engine oil as necessary.
        • If a condition is found, repair as necessary.
        • If no condition is found, inspect the engine for a mechanical condition. Refer to Symptoms - Engine Mechanical.


      •  
     
    • If between −10 % and +10 %

  5. Operate the vehicle within the Conditions for Running the DTC. You may also operate the vehicle within the conditions that you observed in the Freeze Frame/Failure Records data.
  6. Verify DTC P0171, P0172, P0174, or P0175 is not set.
    • If any of the DTCs set
      Refer to conditions listed above.
     
    • If none of the DTCs set

  7. All OK.

Sounds like improper fueling. Fix that first, the P0172 and P0175 codes. You've probably smoked the catalytic converters from running too rich.

 

One of three likely problems (in no particular order):

1. Fuel injectors. 

2. High pressure fuel pump.

3. Low pressure fuel pump.

 

Perform a fuel injector balance test and check/monitor fuel rail pressure (high pressure pump performance) and fuel pump pressure (low pressure pump performance). 

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