The cold start valve is sometimes called the fifth injector. It is mounted to the intake air distribution box and is in line with the fuel rail. It has an electrical connection and a fuel inlet line and a fuel outlet line.
The cold start valve is essentially constructed the same as the regular fuel injectors. The main difference is that the cold start valve injection time is measured in seconds rather than in milliseconds like the regular injectors.
The cold start valve receives power from the double relay when the key is in the start position but only injects fuel into the intake air distributor when the cold start valve is grounded by the thermo-time switch.
The purpose of the cold start valve is to inject fuel into the intake air stream when the engine is cold (and the starter operating) in order to richen the fuel/air mixture for easier starting.
It can be tested in the same manner as the other injectors. Leakage is undesirable as it will cause the engine to run rich.