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Diagnostic Trouble Codes (DTCs)

Published on January 5th, 2024

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Go to our DTC Library to get code descriptions for generic and manufacturer-specific codes.

Diagnostic Trouble Codes

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Diagnostic Trouble Codes (DTCs) are meant to guide you to the proper service procedure in the vehicle’s service manual. Do not replace parts based only on DTCs without first consulting the vehicle’s service manual for proper testing procedures for that particular system, circuit, or component.

DTCs are alphanumeric codes that are used to identify a problem that is present in any of the systems that are monitored by the onboard computer (PCM). Each trouble code has an assigned message that identifies the circuit, component or system area where the problem was found.OBD2 diagnostic trouble codes are made up of five characters:

  • The 1st character is a letter (B, C, P, or U). It identifies the Main system where the fault occurred (Body, Chassis, Powertrain, or Network).
  • The 2nd character is a numeric digit (0 thru 3). It identifies the Type of code (Generic or Manufacturer-Specific).

       - Generic DTCs are codes that are used by all vehicle manufacturers. The standards for generic DTCs, as well as their definitions, are set by the Society of Automotive Engineers (SAE).

       - Manufacturer-Specific DTCs are codes that are controlled by the vehicle manufacturers. The Federal Government does not require vehicle manufacturers to go beyond the standardized generic DTCs in order to comply with the new OBD2 emissions standards. However, manufacturers are free to expand beyond the standardized codes to make their systems easier to diagnose.
  • The 3rd character is a letter or a numeric digit (0 thru 9, A thru F). It identifies the specific system or sub-system where the problem is located.
  • The 4th and 5th characters are letters or numeric digits (0 thru 9, A thru F). They identify the section of the system that is malfunctioning.

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Emissions Fault Diagnosis Table (Example)

Symptom:Possible Cause:
EGR valve sticking-Build-up of carbon
-Electrical fault
High CO and high HC-Rich mixture
-Blocked air filter
-Damaged catalytic converter
-Engine management system fault
Low CO and high HC-Misfire
-Fouled plug(s)
-Weak mixture
Low CO and low or normal HC-Exhaust leak
-Fouled injector
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Note: Emissions Systems Diagnosis Chart.

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