DTC (diagnostic trouble codes) explained

DTCs, commonly referred to as "fault codes", play a crucial role in modern automotive diagnostics. These alphanumeric codes are generated by on-board diagnostic (OBD) systems to flag potential problems in the various control units fitted to a vehicle.

Understanding these codes is essential both for the technician working on the vehicle and for the person responsible for the file. They provide valuable information about the vehicle's health and help identify underlying issues.

These codes usually consist of a letter followed by four digits (e.g. P0420). Each code corresponds to a specific problem or malfunction, making issues easier to identify and resolve during diagnostic procedures.

Generic and manufacturer-specific DTCs

There are two types of DTCs:

Generic DTCs: as their name implies, these codes describe generic faults and do not always give details specific to each vehicle model.
Manufacturer-specific DTCs: in addition to generic codes, car manufacturers can implement their own proprietary DTCs to provide more specific information about their systems and components. These codes can give deeper insight into the root cause of a problem, making diagnostics and repairs more effective.

How to read a DTC

Fault codes must comply with the OBD standard. Here is how to interpret them.

The first letter indicates the fault code family:

P (Powertrain): the powertrain, meaning the engine or transmission.
C (Chassis): parts outside the passenger compartment, such as brakes, steering or suspension.
B (Body): parts inside the vehicle's passenger compartment.
U: codes starting with "U" are generally associated with communication problems between the vehicle's electronic modules. They often point to issues with the network, connectivity or communication between the control units and the various vehicle systems.

The first digit after the letter indicates whether the fault code is generic or not:

0: generic fault
1: manufacturer-specific fault

The last three digits correspond to:

0, 1 and 2: air/fuel mixture control
3: ignition system
4: auxiliary emissions control
5: engine idle control
6: actuators, sensors or control devices sending information to the relevant ECU
7-8: transmission

Why diagnose before and after tuning

In engine tuning, it is imperative to run a vehicle diagnostic before any work, to detect possible anomalies, and again after writing the file. A flash can leave faults behind that were not erased by the interface used to read and write the file.

An efficient multi-brand diagnostic tool is indispensable and allows much more precise work.

The DTC OFF option

BOBPerformance offers an option called DTC OFF. It removes a fault stored in the engine control unit (P fault). This removes the fault, and therefore a possible engine warning light on the dashboard, but it does not fix any mechanical issue.

We recommend this option when everything is mechanically in order on the vehicle but, despite all repairs, the warning light and the faults keep coming back. A DTC OFF cannot fix a vehicle that goes into safe mode because of a recurring fault, for example. In that case, the issue must first be resolved mechanically.

There are several types of DTCs:

Standard DTCs: they only warn of an anomaly and have no effect on engine management (e.g. safe mode). These DTCs can generally be deleted without consequences (note that this does not mean the potential failure disappears).
DTCs related to functions or monitoring: these are known as "major" faults, linked to significant failures (ignition, turbo, air/fuel ratio). You cannot delete them, as this would cause further cascading faults and new related failures.

The DTC removal tool offered by BOBPerformance should be used with caution. We do not recommend deleting significant faults to hide serious mechanical failures, as the long-term effects can be harmful. For example, if you delete a fault from a defective lambda sensor, the problem will still be there, and an air/fuel ratio issue can quickly damage the engine internals.