Multiple random misfires and flashing sel

Misfire codes are very hard to diagnose and correct a pro does this best as they deal with it all the time. Just because it says knock sensor may not be the case. Unfortunately you'll have to take it back and let him scan it again maybe something might be an internal problem with whatever sensor it is or something making the knock sensor go off.

Then go to another place because the 800 dollar repair may be an internal engine problem. Like I said misfires are very hard to pinpoint.

Have an autparts scan it and get back to me they don't charge for it bbut it may be an intermittent due to the flashing light. Give me the code it gives.

True if you have a vacuum leak (with a bad manifold gasket) it will cause a misfire normally at lower rpm then smooths out as you go faster. If you spray choke cleaner by the gasket on both sides that would tell you if it's leaking at top. Like I said these are hard to find.