Hi guys,

First time poster here. I've scoured all over the internet trying to find a solution for a problem I'm having, but I'm none the wiser. Hope someone here might be able to help.

My 2002 Civic (D16V1 engine) is throwing a CEL for some time now. It first started as a P0303 code (misfire in cylinder 3), but after a while, it spread to other cylinders. The strange thing is, I haven't noticed any performance issues. The engine seems to run fine, with plenty of power. The only hint of a problem may be a slightly rough idle at times. Also, I reset the CEL before a recent 1000 km trip, and at highway speeds at over 3k rpm, it didn't come on at all. When driving in the city, it occasionaly flashes at low rpm in high gear, and stops flashing as soon as I downshift and rev the engine a bit.
Here's a list of things I've done so far:
- replaced spark plugs
- adjusted valve clearance
- changed the fuel filter
- coil packs are fine (checked with multimeter, resistance is consistent)

None of the things above seemed to have alleviated the problem.
At one point, I had the engine shutting down during extended rush hour traffic. This was diagnosed as a bad camshaft position sensor, which was replaced. I asked my mechanic to replace both the camshaft and the crankshaft sensors (hoping the latter would solve the misfire), but the strangest thing happened - when he put in both new sensors, the engine wouldn't start at all. Only after installing the old crankshaft sensor did it work again. Is it possible I'd bought a dud?

So far, the only thing I haven't checked are the fuel injectors. But before giving those a go, I just wanted to check with folks much more knowledgeable than myself if there's something blatantly obvious I'm missing here.
Thanks in advance!​​