When I saw Check Engine sign on my dash, I took my van(160k) to a Toyota dealer and they came out with OBD codes for defective front pipe and knock sensor. Before that I had leaky muffler too. They replaced muffler and front pipe thinking that these defective parts triggered 'defective knock sensor OBD code'. But, Check engine sign came again right after replacing parts. Should I go ahead with knock sensor replacement? can someone share opinions on this?