Update - Thanks to Phil (Mercedesshop.com), I got hold of an image of knock sensor !

Came home from work, disconnected wiring harness that was attached to the K.S module and started the engine. I expected the check engine light to come on, but to my surprise, no light. I even used a long screw driver and tap on the engine block to emulate pinging . Nothing.....

Next step - will drive the car and see what happens.