Diagnostic Trouble Codes (DTCs) are codes that identify a specific problem area.

Diagnostic Trouble Codes (DTCs) are numeric (OBD1 systems) or alpha-numeric (OBD2 systems) codes that are used to identify a problem that is present in any of the systems that are monitored by the vehicle’s on-board computer, also called the Powertrain Control Module (PCM).

DTCs are intended as a guide to the proper diagnostic/service procedures in the vehicle’s repair manual, and not as the sole source for repair. DO NOT replace parts based only on DTCs without first consulting the vehicle’s service manual for proper testing procedures for the associated system, circuit or component.

Malfunction Indicator Lamp (MIL)

Every vehicle’s instrument panel is equipped a Malfunction Indicator Light (also called a "Check Engine" light or "Service Engine Soon" light). When the vehicle's on-board computer detects a failure in an emissions-related component or system, the computer's internal diagnostic program

  • Assigns a diagnostic trouble code (DTC) that points to the system (and subsystem) where the fault was found
  • Saves the code in the computer's memory, and
  • Lights the Malfunction Indicator Lamp (MIL).


In OBD1 systems, Diagnostic Trouble Codes (DTCs) are manufacturer-specific. This means that each vehicle manufacturer developed its own set of DTCs, with meanings that apply only to vehicles made by that manufacturer. OBD1 DTCs are generally 2- or 3-digit numeric codes that represent a specific emission-related problem. OBD1 systems use the vehicle’s Malfunction Indicator Lamp ()MIL) to transmit DTCs from the vehicle’s Powertrain Control Module (PCM) by blinking “on” and “off.”

There are two types of fault codes possible in OBD1 systems:

  • “Hard” Codes – “Hard” codes represent problems that are occurring now and cause the MIL to illuminate and remain on until the failure is repaired. A DTC is stored in the computer’s memory for later access.
  • “Intermittent” Codes – “Intermittent” or “pending” codes may cause the MIL to “flicker” or to stay on until the intermittent malfunction goes away. However, the corresponding DTC will be stored in the computer’s memory for later access. If the malfunction does not reappear within a predetermined length of time (normally measured by ignition key start cycles), the computer automatically erases the DTC.


A significant benefit in OBD2 systems is the standardization of Diagnostic Trouble Codes (DTCs). OBD2 DTCs can include both generic and manufacturer-specific codes.

Generic DTCs are codes that are used by all vehicle manufacturers. The definitions are the same, regardless of vehicle make or model. The standards for generic DTCs, as well as their definitions, are set by the Society of Automotive Engineers (ref. SAE J2012) and/or the International Standards Organization (ref. ISO 15031-6).

Manufacturer-specific DTCs are codes that are defined and controlled by the vehicle manufacturer. Vehicle manufacturers are not required to provide manufacturer-specific DTCs in order to comply with OBD2 emissions standards. However, manufacturers are free to expand beyond the required codes to make their systems easier to diagnose.

All OBD2 DTCs are made up of five alpha-numeric characters.


  • The first character is a letter. It identifies the “main system” where the fault occurred.

B - Body
C - Chassis
P - Powertrain
U - Network

  • The second character is a numeric digit. It identified the “type” of code.
0 - Generic codes
1 - Manufacturer-specific codes
2 - Includes both Generic and Manufacturer-specific codes
3 - Includes both Generic and Manufacturer-specific codes

  • The third character is a numeric digit. It identifies the specific system or sub-system where the problem is located.
1 – Fuel and Air Metering
2 – Fuel and Air Metering (injector circuit malfunction only)
3- Ignition System or Misfire
4 – Auxiliary Emission Control System
5 – Vehicle Speed Control and Idle Control System
6 – Computer Output Circuits
7 – Transmission
8 – Transmission

  • The fourth and fifth characters are numeric digits. They identify the section of the system that is malfunctioning.

    OBD2 systems do not use the MIL to transmit codes from the vehicle’s computer. Codes are retrieved from OBD2 systems using a Code Reader or Scan Tool.

There are two types of DTCs used for emissions-related faults in OBD2 systems: Type “A” and Type “B.” Type “A” codes are “One Trip” codes. Type “B” codes are usually “Two Trip” codes.
When a Type “A” DTC is found on the First Trip, the following events take place:
• The computer commands the MIL “on” when the failure is first found.
• If the failure causes a severe misfire that may cause damage to the catalytic converter, the MIL “flashes” once per second. The MIL continues to flash as long as the condition exists. If the condition that caused the MIL to flash is no longer present, the MIL will light steady “On.”
• A DTC is saved in the computer’s memory for later retrieval.
• A “Freeze Frame” of the conditions present in the engine or emission system when the MIL was commanded “On” is saved in the computer’s memory for later retrieval. This information shows fuel system status, engine load, coolant temperature, fuel trim value, MAP vacuum, engine RPM and DTC priority.
When a Type “B” DTC is found on the First Trip, the computer sets a Pending DTC, but the MIL is not commanded “On.” “Freeze Frame” data is not recorded at this time. The Pending DTC is saved in the computer’s memory for later retrieval.
• If the failure is found on the second consecutive Trip, the MIL is commanded “On” and “Freeze Frame” data is saved in the computer’s memory.
• If the failure is not found on the second Trip, the Pending DTC is erased from the computer’s memory.

The MIL will remain “On” for both Type “A” and Type “B” codes until one of the following conditions occurs:
• If the conditions that caused the MIL to light are no longer present for the next three consecutive trips, the computer automatically turns the MIL “Off” if no other emissions-related faults are present. However, the DTCs remain in the computer’s memory for 40 warm-up cycles (80 warm-up cycles for fuel and misfire faults). The DTCs are automatically erased if the fault that caused them to be set is not detected again during that period.
• Misfire and fuel system faults require three trips with “similar conditions” before the MIL is turned “Off.” These are trips where the engine load, RPM and temperature are similar to the conditions present when the fault was first found.
After the MIL has been turned “Off,” DTCs, Freeze Frame data, and manufacturer-specific enhanced data stay in the computer’s memory. This data can only be retrieved by suing equipment such as a Scan Tool.
• Erasing the DTCs from the computer’s memory can also turn the MIL “Off.”
If a Code Reader or Scan Tool is used to erase DTCs from the computer’s memory, Freeze Frame data as well as other manufacturer-specific enhanced data is also erased.