Modern vehicles are a marvel of accomplishment in computer controlled engine system monitoring, management and operation, however, all of that technology comes at a price. Vehicles are more expensive to purchase, maintain and repair due to the inherent complexity with meeting more stringent regulations for fuel economy and emissions in a more compact package with less weight while maintaining performance and improving vehicle longevity. A Powertrain Control Module (PCM) is the brain of your vehicle analyzing individual input variables from sensors then utilizing output controls for the solenoids, valves and actuators of the engine, transmission and body components while monitoring the results. We will focus on the engine control portion of the PCM and discuss cause and effect relationships of the engine management system inputs and the PCM outputs which incorporates On-Board Diagnostics (OBD-II), Diagnostic Trouble Codes (DTC) and the Check Engine Light (CEL). You may also see references to Service Engine Soon (SES) or Malfunction Indicator Light (MIL) which is the same terminology depending on the vehicle design and date of manufacture.

Air, Fuel and Ignition are always the starting points for evaluation and in that order, whether or not a CEL is present, while Combustion and Emission are the result of the Air-Fuel-Ignition process but mechanical engine wear incrementally influences the outputs. Each piece of the puzzle has its own sensors to provide data to the PCM plus individual component controls, however, none of the systems are truly independent as each relies on the other for proper operation. Additional engine specific sensors are also in place to monitor crankshaft position, camshaft timing, engine knock, cylinder misfires and coolant temperature among other things. Up to 65 sensors are incorporated vehicle-wide with thousands of generic Environmental Protection Agency and vehicle specific Original Equipment Manufacturer DTC’s. A DTC is stored in the PCM in the Pxxxx format once computer data input analysis and/or output evaluations indicate an abnormal or unexpected condition and consequently trips the CEL. Multiple DTC’s may be present upon CEL activation with the PCM recording freeze-frame data for the most severe or most current issue with severity always taking precedence regardless of the sequence or frequency of the abnormality. Most vehicles do not have the capability to store multiple freeze-frame files for review of the conditions present at the time of occurrence.

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A CEL means one or more areas of the system are not operating as expected from the vehicle manufacturer but does not necessarily mean the replacement of parts or components is required to solve the problem. Sometimes identifying, locating and cleaning dirty sensors and components will resolve the issue or replacing a relatively inexpensive gasket, o-ring or tube will fix the problem, however, air and vacuum leaks are notoriously difficult and time-consuming to pinpoint. Other situations can be complicated, difficult and expensive to diagnose and repair depending on the system, component or part affected and the engine compartment location especially with multiple DTC’s which are not related. Upstream faults must be addressed first as downstream issues might be the consequence of upstream conditions. A new DTC with a CEL immediately following a repair is not uncommon as accurate data and computer analysis of operating parameters now allows the PCM to report a previously undetectable fault which is a separate service evaluation. A CEL indicates consecutive testing cycle failures, multiple abnormalities within a specified time-frame,exhaust emissions exceeding federal specifications, misfires that can cause damage to the catalytic converter or severe knocking which can damage the engine. Potential damage detection events will cause the CEL to continuously flash on/off while in progress. A CEL should always be properly investigated, diagnosed and repaired before additional costs are incurred that could have been avoided not to mention the wasted fuel, poor performance and pollution.

Case study example of a vehicle with similar DTC triggering events at a two month interval involving oxygen sensors. Oxygen sensors have 81 generic DTC’s in addition to vehicle specific DTC’s. Most people have anxiety at this point alone in the evaluation due to the costs associated with oxygen sensors and catalytic converters, however, a malfunctioning oxygen sensor would have been the cheaper repair in both events for this vehicle.

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1st Event P0171 DTC – Bank 1 Fuel System Too Lean

Lean = Too Much Air or Not Enough Fuel

  • Lack of Power on Acceleration
  • Rough Idle or Misfire
  • Poor Fuel Economy

Usually not Defective, Contaminated or Malfunctioning Oxygen Sensors

  • Defective or Dirty MAF Sensor
  • Defective or Dirty IAT Sensor
  • Defective MAP Sensor
  • Vacuum or Air Leaks
  • Defective or Dirty Fuel Injectors
  • Defective or Dirty EGR Valve
  • Plugged Fuel Filter or Weak Fuel Pump
  • Defective Fuel Pressure Sensor or Regulator

Observations and Data Monitoring

  1. Low Power on Acceleration was Present
  2. Rough Idle was Present
  3. Poor Fuel Economy was Confirmed = STFT +7.5% and LTFT +20%
  4. No MAF Sensor for this Vehicle
  5. IAT Sensor value +20°F
  6. MAP Sensor within Specifications
  7. Air Intake Manifold Pressure with Slight Random Fluctuations at Idle
  8. EGR Valve with Random Errors
  9. Fuel Pressure within Specifications
  10. Ignition Timing was Unusually Retarded
  11. IAC Valve within Operating Parameters
  12. TPS within Operating Parameters

Visual Inspections and Tests

  1. Oxygen Sensors within Operating Specifications
  2. Air Intake Tube was Intact
  3. IAT Sensor was Dirty
  4. Throttle Body coated with Oil Residue
  5. Air Intake Manifold Vacuum with Fluctuations at Idle
  6. EGR Valve was Dirty
  7. STFT and LTFT Improves as RPM Increases

Conclusion is Unmonitored Air or Vacuum Leak

  1. Replaced Faulty Valve Cover Gasket
  2. Cleaned IAT Sensor
  3. Cleaned Throttle Body
  4. Cleaned Intake Manifold and Fuel Injectors
  5. Cleaned EGR Valve
  6. Replaced Ignition Coil Pack and Spark Plug Wires

Firing voltage below specifications on cylinders #1 and #4 (waste spark system) as electrical arcing damaged the coil and wires due to pooled oil in the plug recess holes as a consequence of the failed valve cover gasket.

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2nd Event P2096 DTC – Post Catalyst Fuel Trim System Too Lean

Lean = Too Much Air or Not Enough Fuel

  • Lack of Power on Acceleration
  • Rough Idle or Misfire
  • Poor Fuel Economy

Could be Defective, Contaminated or Malfunctioning Oxygen Sensor or Catalytic Converter

Observations and Data Monitoring

  1. Low Power on Acceleration was Present
  2. Rough Idle was Present
  3. Poor Fuel Economy was Confirmed = STFT 0% and LTFT +16%
  4. IAT Sensor within Specifications
  5. MAP Sensor within Specifications
  6. Air Intake Manifold Pressure within Specifications
  7. EGR Valve within Specifications
  8. Fuel Pressure within Specifications
  9. Ignition Timing within Specifications
  10. IAC Valve within Operating Parameters
  11. TPS within Operating Parameters

Visual Inspections and Tests

  1. Oxygen Sensors within Operating Specifications
  2. Upstream Oxygen Sensor Volts, Amplitude and Frequency within Specifications
  3. Downstream Oxygen Sensor Biased Lean with Occasional Values near 0 Volts and Sporadic Amplitude Fluctuations
  4. Catalytic Converter Intact and Operational
  5. LTFT Improvement as RPM

Conclusion is Air Leak Near Downstream Oxygen Sensor

  1. Replaced Front Exhaust Pipe

Pipe following the catalytic converter with the downstream oxygen sensor inserted into the pipe immediately following the flanged connection to the catalytic converter. Air was continuously entering the exhaust pipe due to a cracked weld located around the oxygen sensor mount and grossly affected the reported exhaust gas emission values at times of low exhaust back-pressure.

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