kia rio 2002 intermiitent stop

I have a KEA RIO AUTOMATIC 2002 CAR and it stops when speed is lowered and engine light comes on .And it starts again with no light it is started.Some times it stops at red light because it is not moving ,there is some******** as thereis toruble in gas flow.
My mechanic recommended fuel injection cleaning ,and after cleaning problem is still there.Diagonsitc codes tells knock sensor failure ,but mechanic says it may not be aproblem.
please help