My car has 66000 miles ( I commute 150 miles) and has been perfect until the warranty ran out. I started getting spark knock about 3 months ago. Then shortly after it would misfire ( later I would discover mostly cylinder 4) but only after about 100 miles of driving. And it never threw a code. The dealership could only find a bad catalytic converter which they replaced. They told me something else was causing it to misfire and probably dumping fuel into the cat and ruined it. But they did not know why. The next shop checked the compression and replaced all the coil packs and plugs with NGK iridium plugs. Ran fine for around 110 miles then cut off and will turn over fine but won't start. Now it has a 304 misfire code and no spark to any coils.