How are OBD-II codes formatted?

OBD-II codes follow a 5-character format: The first letter indicates the system — P for Powertrain, B for Body, C for Chassis, and U for Network/Communication. The second character (0-3) indicates whether it's a generic SAE code (0) or manufacturer-specific code (1). The third character identifies the specific subsystem (e.g., fuel system, ignition). The fourth and fifth characters pinpoint the specific fault condition. For example, P0171 means: P=Powertrain, 0=Generic SAE, 1=Fuel/Air system, 71=System too lean Bank 1.