What is the severity of DTC in automotive ECU diagnosis?

What is DTC severity?

The DTC severity occupies 1 byte of data and contains two parts of information, DTC severity information (3 bits) and DTC category information (5 bits), as shown below:

source: ISO15031-6

DTC severity information refers to the impact or severity of a specific diagnostic trouble code. It indicates the severity of the fault and helps prioritize the necessary actions to diagnose and deal with the problem. Severity usually classifies the potential consequences and impacts of the identified fault, which are divided into three categories in ISO14229-1 as follows:

source: ISO15031-6

Maintenance Only: When a diagnostic code or warning indicates a non-urgent problem or a routine maintenance task that requires attention, but no immediate action is required. It indicates that the problem should be addressed at the next scheduled maintenance or service appointment.

CheckAtNextHalt: This means that a fault or problem should be checked or resolved as soon as it is safe to do so, usually at the next stop. While the problem may not be an immediate safety issue, early inspection is recommended to prevent potential harm or further damage.

Check Immediately: means that the diagnostic code or warning indicates that there is a critical or serious problem that requires immediate attention. It indicates that the vehicle should be stopped as soon as it is safe to do so and the necessary steps should be taken to resolve the problem promptly. Continuing to operate the vehicle without resolving the problem could pose a safety risk or cause further damage.

The exact definition and severity will vary depending on the manufacturer and the diagnostic system used.

DTC level information refers to the classification or grading of DTCs according to the affected systems or components, and is applicable to OBD systems that comply with WWH-OBD GTR. Class A, B1, B2 or C are the attributes of emission-related DTCs, which describe the impact of the fault on emissions or on the monitoring capabilities of the OBD system, and are defined as follows: