Three diagnostic approaches can be used to solve driveability problems: code-based diagnostics, symptom-based diagnostics, and system-based diagnostics. Each has a specific purpose and should only be undertaken as directed by the On-Board Diagnostic (OBD) check.
Code-Based Diagnostics
Code-based diagnostics centers around the Diagnostic Trouble Codes (DTCs) stored by the PCM. These codes are systematically numbered so that each code relates to a specific fault at a specific component. Using a “Scan” tool or the Malfunction Indicator Lamp (MIL), you can identify and correct the code condition(s) by following the appropriate trouble tree.
As much as possible, code numbering consistency is used in all GM vehicles. Depending on vehicle options and equipment, code numbers may or may not apply to specific vehicles. When using code-based diagnostics, always begin with the lowest numbered code first.
A complete list of 6.5L EFI diesel DTCs is contained in section 7 of this book. The list includes the code number, name, and the page number on which the circuit description for the code is located.
Symptom-Based Diagnostics
Symptom-based diagnostics are used for conditions that don’t set Diagnostic Trouble Codes but still result in driveability complaints such as sluggish performance, hard starting, or intermittents. These conditions generally produce normal “Scan” tool data values. Procedures for diagnosing driveability symptoms are found in section 2, “Driveability Symptoms,” of the Driveability and Emissions manual.
Covered conditions include:
• Intermittents
• Hard Starts
• Surges and/or Chuggles
• Lack of Power, Sluggish, Spongy
• Fuel Knock/Combustion Noise
• Poor Fuel Economy
• Excessive Smoke
