These names cause endless confusion! There are 3 components underneath the inlet manifold. The one at the front, with one connection, is the sensor for the temperature gauge on the instrument panel. That is its only function. Behind that is the knock sensor, which you correctly identify. Then, roughly at no. 3 cylinder, is another temperature sensor, with 2 connections, used by the engine management system. It is difficult to see and even harder to remove, and it is sometimes recommended that you remove the inlet manifold first for access. Cars without this system, like basic carburettor models, just have a blanking plug here.
I hope this explanation helps. You have bought the rearmost sensor, but which you need depends on what the trouble is.