What do the letters P, B, C and U mean at the start of a code?
Short answer: P is powertrain, B is body, C is chassis and U is network communication; the letter tells you which family of systems set the code.
Every OBD-II code starts with a letter that names the system group. P codes cover the engine, transmission and emissions. B codes cover body systems like airbags, seats, lighting and climate. C codes cover chassis systems such as ABS, traction control and steering. U codes cover the communication network between modules.
The second character tells you who defined it. A 0 means a generic SAE code with the same meaning on every vehicle, such as P0300. A 1 usually means a manufacturer-specific code, so P1xxx on a Ford and P1xxx on a Toyota can mean completely different things. Some ranges in P2 and P3 are split between generic and manufacturer use.
That is why a generic code reader can show P0 codes but often misses B, C and U codes and many manufacturer codes. A scan tool with enhanced, manufacturer-level coverage reads all of them and shows the OEM description.
Tools used for this job
General guidance only. Follow the vehicle and tool manufacturer's procedures, and confirm vehicle coverage on the tool maker's list before buying.