Freightliner DTC Codes are essential diagnostic tools that help identify faults in heavy-duty trucks and equipment powered by Detroit Diesel engines. By understanding the SPN and FMI structure, technicians can quickly pinpoint issues in fuel, air, electrical, transmission, and safety systems without guesswork. This guide covers the most critical and commonly encountered codes, along with their symptoms, causes, and practical repair steps. From low fuel rail pressure and coolant temperature warnings to CAN communication errors and ABS system faults, each code provides insight into how the machine is performing and what needs immediate attention. By following a structured troubleshooting approach and prioritizing repairs based on severity, operators and fleet managers can reduce downtime, prevent costly failures, and maintain reliable equipment performance in demanding work environments.