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JOHN aka Eauto, The Car Guy
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2003 chevy: Z71..P0171..lean..Code p300 multiple cyclinde misfire

Resolved Question:

2003 chevy 1500 Z71 have code P0171 bank to lean what does this mean and how do I fix it? Code P0300 multiple cyclinde misfire how do I fix? Code P0420 Cat converter below threshholdbank1 of 4 how do I fix?
Submitted: 7 years ago.
Category: Chevy
Expert:  JOHN aka Eauto replied 7 years ago.

I would start by checking for vacum or air intake leaks

i will also supply you with the flow chart for diagnostics and a service bulletin

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The Powertrain Control Module (PCM) controls a Closed Loop air/fuel metering system in order to provide the best possible combination of driveability, fuel economy, and emission control. The PCM 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. You can use a scan tool in order to monitor the short and long term fuel trim. The ideal fuel trim values are around 0 percent . A positive fuel trim value indicates that the PCM is adding fuel in order to compensate for a lean condition. A negative fuel trim value indicates that the PCM is reducing the amount of fuel in order to compensate for a rich condition. If the PCM detects an excessively Rich or Lean condition, the PCM sets a Diagnostic Trouble Code (DTC) . The long term fuel trim diagnostic parameter is an average of several of the long term speed load learn cells which the PCM selects based on the engine speed and the engine load.


  • No active Mass Air Flow (MAF) DTCs
  • No active Manifold Absolute Pressure (MAP) DTCs
  • No active Intake Air Temperature (IAT) DTCs
  • No active Engine Coolant Temperature (ECT) DTCs
  • No active Throttle Position (TP) DTCs
  • No active injector DTCs
  • No active misfire DTCs
  • No active Knock Sensor (KS) DTCs
  • No active crank sensor DTCs
  • No active ignition control DTCs
  • No active Exhaust Gas Recirculation (EGR) DTCs
  • No active secondary Air Injection (AIR) DTCs
  • No active Evaporative Emission (EVAP) DTCs
  • No active Vehicle Speed (VS) DTCs
  • The ECT is between 50°C (122°F) and 115°C (239°F) .
  • The barometric pressure is more than 74 kpa .
  • The MAF is between 5 g/s and 90 g/s .
  • The MAP is between 26 kpa and 90 kpa
  • The IAT is between -20°C (-4°F) and +90°C (+194°F) .
  • The engine speed is between 400 RPM and 3,000 RPM .
  • The TP sensor angle is less than 90 percent .
  • The vehicle speed is less than 137 km/h (85 mph) .
  • The fuel level is more than 10 percent .


  • The average long term fuel trim cell values are above a predetermined threshold.
  • All the above conditions are present for 6 seconds .


  • The PCM illuminates the Malfunction Indicator Lamp (MIL) on the second consecutive ignition cycle that the diagnostic runs and fails. If the diagnostic fails under the same conditions such as load, RPM, temperature, etc. as the previous ignition cycle that the test ran and failed.
  • The first time the diagnostic fails, the PCM records the operating conditions in Failure Records.
  • The second consecutive ignition cycle that this diagnostic fails under the same conditions such as load, RPM, temperature, etc. as the previous ignition cycle that the test ran and failed, the PCM stores this information in Freeze Frame. The PCM copies any data previously stored in Freeze Frame and copies the data into the Failure Records. The PCM then overwrites the Freeze Frame. The only exception to this is if a misfire DTC was already recorded in Freeze Frame. In this case, the misfire data stays in Freeze Frame and the PCM updates the fuel trim data in the Failure Records.


  • The PCM turns the MIL OFF after three consecutive ignition cycles that the diagnostic runs and does not fail within the same conditions that the DTC last failed.

IMPORTANT: If the last failure was during a non-typical driving condition, the MIL may remain ON longer than the three ignition cycles. Review the Freeze Frame and Failure Records for the last failure conditions.

  • A History DTC clears after forty consecutive warm-up cycles, if this or any other emission related diagnostic does not report any failures.
  • A last test failed clears when the diagnostic runs and does not fail.
  • Use a scan tool in order to clear the MIL/DTC.


  • This DTC sets if the engine runs out of fuel.
  • A fuel delivery malfunction causes this DTC to set. Thoroughly inspect all items that cause a lean condition.

The numbers below refer to the step numbers on the diagnostic table.

  1. This step determines whether the fault is present.
  2. Using the Freeze Frame and Failure Records data may aid in locating an intermittent condition. If you cannot duplicate the DTC, the information included in the Freeze Frame and Failure Records data can help determine how many miles since the DTC set. The Fail Counter and Pass Counter can also help determine how many ignition cycles the diagnostic reported a pass or a fail. Operate the vehicle within the same Freeze Frame conditions such as RPM, load, vehicle speed, temperature, etc. that you observed. This will isolate when the DTC failed. For an intermittent condition, refer to Symptoms. See: Symptom Related Diagnostic Procedure
  3. If DTC P0174 is also set, this indicates both banks of the engine are operating lean. Inspect the items that would cause both banks to operate lean.
  4. A vacuum leak causes DTCs P0171 and P0174 to set at the same time. Inspect all areas of the engine for a vacuum leak. Also inspect the PCV valve for being the correct one for this application. Make sure the engine oil fill cap is in place and that it is tight. Verify that the engine oil dip stick is fully seated.
  5. ......
  6. Subject: 97-05 Vehicles with 4.8 5.3 5.7 and 6.0 Gen III V8 Engines and Misfires on One Bank - kw CEL diagnostics driveability DTC P0171 P0172 P0174 P0175 P0300 P0301 P0302 P0303 P0304 P0305 P0306 P0307 P0308 #PIP3056 - (07/12/2004)

    Models: (00-05 Cadillac Escalade - C6 and K6) and (02-05 Chevrolet Avalanche - C1, C2 K1 and K2) and (99-05 Chevrolet Silverado - C1, C2 K1 and K2) and (00-05 Chevrolet Suburban - C1, C2, K1 and K2) and (00-05 Chevrolet Tahoe - C1 and K1) and (99-05 GMC Sierra C1, C2 K1 and K2) and (00-05 GMC Yukon - C1, C2, K1 and K2) and (03-05 Chevrolet Express G1, G2, G3, H1, H2 ) and (03-05 GMC Savana G1, G2, G3, H1, H2 ) and (04-05 Buick Rainier S1, T1 ) and (03-05 Chevrolet Trail Blazer EXT S1, T1 ) and (03-05 GMC Envoy XL XUV S1, T1 ) and (03-05 Hummer H2) and (04-05 Cadillac CTS-V DB) and (98-02 Chevrolet Camaro FB) and (97-04 Chevrolet Corvette YB) and (98-02 Pontiac Firebird FB) and (04-05 Pontiac GTO VB)

    The following diagnosis might be helpful if the vehicle exhibits the described symptoms.

    Check Engine Light with a P0300 due to 2 - 4 cylinders misfiring on the same bank of the engine. DTCs P0171, P0172, P0174 or P0175 may also set for the misfiring bank.

    If the published misfire diagnostic does not isolate the cause, perform the following suggestions as necessary:

    • Perform a fuel injector balance test for all 8 cylinders. If a fuel injector concern exists, it is possible to misfuel an entire bank of the engine, causing multiple cylinders on the same bank to misfire even though the root cause is a single fuel injector.
    • Inspect O2 sensor connections on the misfiring bank for corrosion or water intrusion. If water intrusion is found on the right bank, it may be due to the AC Evaporator Condensation dripping onto the O2 sensor harness. If this condition is found, reposition and shield the harness to prevent a repeat concern and repair the connections.
    • Check for excessive exhaust backpressure using the restricted exhaust diagnosis.
    • Swap the Position 1 O2 sensors side to side to see if the misfires move to the other bank of the engine. If so, replace the O2 sensor.

    Please follow this diagnosis process thoroughly and complete each step. If the condition exhibited is resolved without completing every step, the remaining steps do not need to be performed.

    NOTE: GM bulletins are intended for use by professional technicians, NOT a "do-it-yourselfer". They are written to inform these technicians of conditions that may occur on some vehicles, or to provide information that could assist in the proper service of a vehicle. Properly trained technicians have the equipment, tools, safety instructions, and know-how to do a job properly and safely. If a condition is described, DO NOT assume that the bulletin applies to your vehicle, or that your vehicle will have that condition.



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