Diagnostic Trouble Code (DTC)

A diagnostic trouble code (DTC) is a standardised alphanumeric code that a vehicle's on-board computer stores when it detects a fault in one of its systems, such as the engine, emissions control, brakes or electronics. The code points to the affected system and the type of problem, helping a technician understand what is wrong before opening the bonnet.

How a DTC is generated

Modern vehicles contain several electronic control units that constantly compare sensor readings with expected values. When a value stays outside its normal range, the relevant ECU records a trouble code, may store a snapshot of the conditions at that moment and, for some faults, switches on a warning light on the dashboard. The code can then be read through the on-board diagnostics (OBD) port with a scan tool, or remotely by a telematics system.

How to read a trouble code

Standard OBD codes follow a common structure: a letter followed by four characters.

• The letter identifies the system: P for powertrain, B for body, C for chassis and U for network communication between modules.
• The next character shows whether the code is generic, shared across manufacturers, or specific to one manufacturer.
• The remaining characters narrow down the subsystem and the precise fault.

A code says where a problem was detected, not always what caused it. A misfire code, for example, can come from a spark plug, an injector or a wiring issue, so a garage diagnosis is still needed.

What DTCs change for a business fleet

On a single private car, a warning light is often noticed late. In a fleet, remote access to trouble codes changes the workflow:

• Earlier action: a fault is visible to the fleet team as soon as it is reported, not when the driver mentions it.
• Prioritisation: severe codes justify stopping the vehicle; minor ones can wait for the next planned service.
• Better garage visits: the workshop receives precise information, which shortens diagnosis.
• Traceability: codes, repairs and their resolution build a reliable maintenance history.

How Dadycar helps

Dadycar is a French fleet management software that brings trouble codes into the maintenance workflow. Telematics reports vehicle data and fault codes from connected vehicles, without a box on compatible models and with an optional tracker for the others. Alerts notify the team when a fault is reported, a work order records the repair and its cost, and the result is kept in each vehicle's maintenance history alongside reminders and due dates.

FAQ

Is every DTC an emergency?
No. Some codes indicate a minor or temporary issue, while others, such as faults linked to braking or engine overheating, require stopping the vehicle quickly. Severity must be assessed case by case.

Does clearing a code fix the problem?
No. Clearing only erases the stored code. If the fault is still present, the code will return; a proper repair is needed.

Can fleet managers read DTCs without visiting the vehicle?
Yes, when the vehicle is connected through telematics. Otherwise, a scan tool must be plugged into the OBD port.