Definition of discourse marker noun from the Oxford Advanced Learner's Dictionary
When an engine starts knocking, one of the first things a mechanic will check is the oil pressure. A warning light may come on in the car, and adding sufficient oil will solve the problem. Sometimes, a knocking noise is caused by loose torque converter bolts or a loose flywheel.
It appears that the knock sensor operation is at least partly the cause for the lack of power and check engine light you're experiencing. However we also noted a broken exhaust manifold which can cause both engine noise and lack of power.