What is a DTC?
Error codes are generated by a vehicle’s onboard diagnostic system to help identify mechanical issues or malfunctions. Learn more about how DTCs work and their role in vehicle maintenance
Error codes are generated by a vehicle’s onboard diagnostic system to help identify mechanical issues or malfunctions. Learn more about how DTCs work and their role in vehicle maintenance
DTC stands for Diagnostic Trouble Code. These are codes generated by a vehicle’s onboard diagnostics (OBD) system when it detects an issue or malfunction in the vehicle’s systems, such as the engine, transmission, or other components. DTCs help mechanics and vehicle owners identify and troubleshoot specific problems using diagnostic tools or scanners. Each DTC corresponds to a particular fault, and they typically follow a standardized format.
On-board diagnostic systems in vehicles continuously monitor functions related to the engine, transmission, and emissions control systems. When an issue is detected, a Diagnostic Trouble Code (DTC) is generated, and the "check engine" light on the dashboard illuminates. This light stays on until the problem is diagnosed and resolved, though it may turn on and off if the issue is intermittent.
A DTC is an alert that signals a fault in the vehicle’s system, requiring a diagnostic tool to read and interpret the specific error code. This code helps identify the underlying issue, whether related to engine performance, transmission, or sensors. If no DTC definition is found, it may indicate a more obscure problem. Understanding DTC codes is essential for diagnosing and fixing vehicle problems effectively.
Before the widespread use of Diagnostic Trouble Codes (DTCs), diagnosing vehicle problems was often time-consuming. With the introduction of OBD-II systems, vehicles can now monitor themselves and alert drivers to potential issues using dashboard indicator lights.
These lights signal a variety of warnings, such as:
Some indicator lights, like the brake system light, can indicate multiple issues. For example, it may show that the parking brake is on, the brake fluid is low, or that there’s a problem with the antilock braking system (ABS).
When the check engine or malfunction indicator light (MIL) illuminates, it indicates that the vehicle’s computer has set a DTC. A diagnostic tool is then needed to read and interpret the specific DTC code.
Each DTC consists of five characters. For example, you might encounter P0575. Here’s how to break it down:
The first letter identifies which system is at fault:
P = Powertrain (engine and transmission)
B = Body (interior and exterior components)
C = Chassis (brakes, suspension, steering)
U = Network (communication systems)
The second character tells whether the code is a generic OBD-II code or a manufacturer-specific code:
0 = Generic code
1 = Manufacturer-specific code (used when a generic code doesn't cover the fault)
The third character indicates which vehicle system is affected:
1 = Fuel and Air Metering
2 = Fuel and Air Metering (injector circuit)
3 = Ignition System or Misfire
4 = Auxiliary Emissions Controls
5 = Vehicle Speed Control and Idle Control System
6 = Computer and Auxiliary Outputs
7-9 = Transmission and Gearbox issues
A-C = Hybrid Propulsion
The last two characters pinpoint the exact fault within the system.
For example, in P0575, it’s a generic OBD-II powertrain fault. The specific issue is related to the vehicle’s speed control or idle control system, and further checking would reveal that it’s a problem with the cruise control input circuit.
There are over 5,000 DTC codes—both generic OBD-II and manufacturer-specific codes. Fortunately, you don’t have to memorize them, but certain DTCs come up frequently enough that you’ll recognize them over time.
The DTC code provides valuable insight, allowing mechanics to quickly diagnose problems. If no DTC definition is found on a code reader, it may require additional troubleshooting.