coding, mechanical keying, connector coding, colour coding, error-proofing
A vehicle contains hundreds of connectors, many of them the same family, the same cavity count and the same size. Keying is what stops them being interchangeable. Two features do the work. Polarization prevents a connector being rotated and forced into its correct mating half the wrong way round. Coding prevents a connector being mated into the wrong mating half entirely.
Polarization is usually geometric: an asymmetric housing profile, a rib down one side, a chamfered corner. Coding is usually a set of variants, often labelled Code A, Code B, Code C, where the moulded key positions differ and each code accepts only its counterpart. Manufacturers frequently pair coding with colour, so a black Code A housing and a grey Code B housing are the same part in every other respect and will not mate with each other.
This is where a part that "looks the same" becomes expensive. Within a family, keying and colour variants often differ by a single digit or suffix on the MPN. A housing that is physically identical apart from a two-millimetre rib will assemble onto the wrong branch of the harness, or refuse to assemble onto the right one, and the fault surfaces at end-of-line test, or later, on the vehicle.
Colour has a second job in service. Coding colours let an assembler or a technician tell apart two connectors sitting centimetres from each other on the same module without reading a label. That is a designed-in error-proofing measure, which means a colour substitution is not cosmetic even when the electrical function is identical.
Harness Code treats keying, coding and colour as part of the part identity rather than as options. Where a direct match is unavailable, the coding and colour of any proposed alternative are stated in writing, so the buyer checks it against the drawing instead of assuming it.
Related terms
Connector housing
A connector housing is the moulded plastic body that holds and positions the terminals of an automotive connector. It is sold separately from the terminals and seals it accepts, so a working connection is always a matched set of housing, terminal and seal, not a single part number.
Manufacturer Part Number (MPN)
A Manufacturer Part Number (MPN) is the unique code a component maker assigns to one exact product variant. In automotive connectivity, the MPN fixes housing, cavity count, keying, colour and plating, so two parts with different MPNs are not interchangeable even if they look identical.
Pin and socket (contact gender)
Pin and socket describe the two halves of an electrical contact. The pin, also called male, tab or blade, enters the socket, also called female or receptacle. Contact gender is fixed by the connector housing and by the circuit's function, not by the wire.
CPA / TPA (connector and terminal position assurance)
A TPA (Terminal Position Assurance) is a secondary lock that confirms every terminal is fully seated in its cavity. A CPA (Connector Position Assurance) confirms two connector halves are fully mated and latched. Both are error-proofing parts, and both are separate part numbers.