Fault codes, also known as diagnostic trouble codes (DTCs), are central to modern vehicle diagnostics. These codes are generated by electronic control units (ECUs) when a malfunction is detected in a monitored system. They are essential tools for identifying issues across all systems.

For heavy-duty vehicles, fault codes often follow the SAE J1939 standard, using Suspect Parameter Numbers (SPNs) and Failure Mode Identifiers (FMIs) to convey detailed information about the issue. Also OBD-II standard is used, which use a structured alphanumeric format (P0100 for example). Many systems also use proprietary fault codes represented just by a decimal or hexadecimal numbers. TruckTester decodes all fault codes in text form.

Fault code (DTC) reading

Fault code clearing

Clearing fault codes is a routine procedure performed after resolving the underlying issue. While clearing codes resets the ECU’s fault memory, it also erases freeze frame data and resets adaptive learning functions. This process ensures that only active issues are flagged during subsequent diagnostics. However, clearing codes without addressing the root cause can obscure underlying problems, leading to more significant failures over time.

Fault code freeze frame reading

DTC freeze frame data is a snapshot of the vehicle's operating conditions at the moment a fault was detected and logged in the ECU. This data typically includes parameters such as engine speed, vehicle speed, coolant temperature, fuel system status, and sensor readings like throttle position or manifold pressure. Freeze frames provide invaluable context for diagnosing issues, especially intermittent or transient faults that might be difficult to replicate. For example, in a heavy-duty truck, a freeze frame associated with an SPN for fuel pressure might reveal that the fault occurred during high engine load, suggesting a possible fuel delivery issue. Technicians use freeze frame data to recreate the exact conditions that triggered the fault, enabling more accurate troubleshooting and faster identification of root causes.