SAE J2012
Workshop glossary › Diagnostics & protocols · UpdatedThe SAE standard that defines DTC structure (P/B/C/U + 4 digits) and the meaning of generic codes in the P0xxx, B0xxx, C0xxx, U0xxx ranges.
SAE J2012 standardises diagnostic trouble code format and the list of generic codes. It is the reason a P0301 on a Toyota, a Volvo, and a Ford all mean the same thing: misfire detected on cylinder 1.
Each code has four parts: system letter (P/B/C/U), generic-vs-manufacturer flag (0 = generic, 1 = manufacturer; 2 and 3 are reserved extensions added later), subsystem digit, and fault number. So P0420 is a generic powertrain code in the emissions subsystem with fault 20 (catalyst efficiency bank 1). P1420 would be a manufacturer-defined code with the same numeric pattern and a different meaning per brand.