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Five characters — one letter and four digits — decode to a system, an origin and a specific fault.
A standard diagnostic trouble code is five characters: one letter followed by four digits. The letter names the system — P for powertrain, C for chassis, B for body and U for network — so the first character alone tells you which corner of the car is complaining.
The second character, the first digit, names the author. A 0 means the code is SAE generic, defined in the public standard and readable by any scanner; a 1 means manufacturer-specific, defined by the vehicle maker. P0301 and P1301 sit side by side in many scan logs for exactly this reason: same system, different authorities.
On narrow screens, swipe or scroll the plate sideways.
The number ranges make the boundary explicit. Generic codes occupy P0000-P0999 and P2000-P2999; the block P1000-P1999 is reserved to the vehicle maker. The practical consequence: the same underlying fault can carry a different number on two brands, so a forum post about one make may not transfer to another.
The remaining digits narrow the fault within the system — the sub-system and the specific failure — which is why a generic reference takes you most of the way on a P0 code but leaves you needing the maker's documentation on a P1 code.
Whatever the number, the code is only the headline. The ECU also stores whether it is pending or confirmed and a freeze frame of the moment it set; reading all three together is the subject of Reading, Clearing and the Drive-Cycle Procedure.
Further reading