CPA, TPA, secondary lock, terminal position assurance, connector position assurance, TPA slider
Primary retention in a connector is done by a lance, a small spring finger that catches the terminal as it is pushed home. A lance can be defeated. A terminal that is not pushed fully in will still sit in the cavity, still make contact on the bench, and then push back out under mating force or vibration. That failure is called a back-out, and it is one of the most common causes of intermittent electrical faults in a harness.
A TPA closes that gap. It is a moulded insert or slider that can only reach its final position when every terminal is fully home. If one terminal is short of seat, the TPA will not close, and the assembler knows before the connector leaves the bench. On many families the TPA ships pre-staged in the housing at a pre-lock position and is pushed to full lock after terminal insertion.
A CPA does the same job one level up, on the mating interface. It is a clip or slider that cannot be pushed home unless the two halves are fully latched, and once home it blocks the latch from releasing. Safety-critical circuits, airbag squibs and high-current connections routinely require one.
For procurement, the point is that both are line items. A TPA has its own MPN, its own colour variants, sometimes its own coding, and its own lead time, entirely independent of the housing it fits. It is cheap, easy to overlook on a BOM, and perfectly capable of stopping a build on its own. Sourcing a housing without confirming the matching secondary lock is a half-solved shortage.
Harness Code quotes secondary locks and position assurance parts alongside the housing they belong to, and confirms the correct variant rather than assuming a family has only one.
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.
Terminal / contact
A terminal, or contact, is the metal part crimped to a wire and inserted into a connector housing to make the electrical connection. Terminals are specified by family, gender (pin or socket), wire cross-section range, and plating, typically tin or gold.
Keying and polarization
Keying, or polarization, is the mechanical feature that stops a connector being mated the wrong way round or into the wrong mating half. Coding variants and colour coding distinguish otherwise identical connectors so a harness cannot be assembled or serviced incorrectly.
Approved Vendor List (AVL) / Bill of Materials (BOM)
A Bill of Materials (BOM) is the complete list of parts and quantities needed to build a product. An Approved Vendor List (AVL) names the manufacturers and suppliers a buyer is permitted to source those parts from. The BOM says what; the AVL says from whom.