DTC (diagnostic trouble codes), also known as OBDII codes, are your car's system for alerting you of vehicle issues. There are typically limits for each system within your vehicle. When the vehicle detects that the issue exceeds these limits, it sends out a trouble code. Technicians use these codes to diagnose and repair vehicle issues. Trouble codes can tell you about any system issues within your vehicle, including airbags and brakes. And since all vehicles since 1996 have OBDII diagnostics, it's unlikely you don't have it. The challenge with this is that every model has different codes and meanings. For instance, a Ford won't have the same codes as a Chevrolet. This makes it difficult if you're reading the codes on your own unless you know the codes for your specific model. In addition, the methods for repairing these issues are likely different for each make and model. Knowing the codes for your fleet vehicles is essential to proper maintenance and repair. The Four Systems of DTC: Before DTC, diagnosing a vehicle came down to engine misfire and gross failures of parts. The advent of OBDII allowed vehicles to monitor their own systems and alert drivers as issues arise with indicator lights. Most of us recognize the most common ones: the check engine light and the oil change indicator. Today, our vehicles provide us and technicians with more information than ever before with DTC. The Society of Automotive Engineers created the original list of codes, which all vehicle manufacturers had to adapt to comply with emission regulations. European and Asian automakers adopted the same list. These codes are categorized into four main systems: B-codes (body codes): Though the body often refers to the outside of the vehicle, B-codes refer to features inside. More include comfort, convenience, and safety features. C-codes (chassis): Functions outside of the passenger compartment including brakes, steering, and suspension. U-codes (network/vehicle integration): Functions shared among the vehicle's systems and computers. P-codes (powertrain): Include functions such as the engine, transmission, and drivetrain. from azuga.com