How OBD-II Codes Are Built: P, C, B, U and the Digits
Four families
The first letter says which system the fault sits in. P is powertrain, the engine and transmission. C is chassis, brakes and steering. B is body, airbags, climate and comfort. U is network, the modules talking to each other.
The first digit: standard or manufacturer
A zero after the letter means the code is standard. The same P0420 means the same thing on a Ford, a Honda and a Hyundai, because it comes from the SAE list every maker uses. A one means the maker wrote it. P1349 on a Toyota is Toyota's own definition and another brand will use that number differently.
The second digit: the system inside the family
For P codes the second digit narrows it: fuel and air metering, fuel and air metering again for injector circuits, ignition and misfire, auxiliary emissions controls, speed and idle control, computer outputs, then transmission. The same idea applies inside the C, B and U families.
The last two digits: the fault
They name the individual circuit or failure. P0420 is powertrain, standard, emissions, fault 20: catalyst efficiency below threshold on bank 1. P1349 is powertrain, manufacturer, misfire group, fault 49: on Toyota that is the VVT system on bank 1.
Why the digits matter to you
If the first digit is a zero, the code is shared with every other make and the generic fault list applies. If it is a one, you need the brand. That single digit decides which lookup is worth your time, and it is why this site carries all 9415 generic codes while the brand sites carry the maker-specific numbers.
Related
How to read OBD-II codes, stored versus pending versus permanent. Connector pinout. The code data as CSV and JSON.