My '01 ex keeps throwing a p0303 code, and sometimes has a pending code for catalysy system below threshold. I've replaced the plugs with NGKs and replaced all of the coil packs. The car does not overheat or have coolant in the oil. What could be other causes for this problem, it only seems to effect cylinder 3. Thanks for the help!

 

I had those issues for a while and kept swapping out coils which seemed to help but misfires would return. I got a nice group of four off a pull a part car that just got put out on the yard. Then I decided to check the almost new plugs and found a loose one. This damaged the plug but gave me other misfire codes for some reason. Replaced it, used proper torque spec on plugs, put nice clean set of oem coils in (returned extra ones) and haven't had any issues last couple of years.

 

Update: now has multiple cylinder misfires. I ran a compression test and its 180-190 psi across the board. Also double checked valve lash and everything checked out. Could a clogged egr system cause misfire codes and not an egr code? Or possibly a camshaft or crankshaft position sensor or O2 sensor? Running out of ideas on my end.

 

Have you tried to check resistance in the fuel injector or swamping a fuel injector. Do you have any exhaust leaks ??

 

I haven't checked resistance yet but i have unplugged the injectors individually they all seem to be working. There is a small hole in the end of the muffler.

 

Usually when you get the catalytic converter below threshold code; it’s a sign of a bad cat. I would replace cat and both 02 sensors before diagnosing the misfires because that can for sure cause vehicle to misfire since it’s messing with the fuel regulation.

 

Don't replace Cat Conv until you get the misfires resolved.
Fuel pump failing - check fuel pressure?
You already checked the cylinder compression for failing piston rings.
Injectors, coils, plugs, poor contacts at the coil and injector connectors, open or short in wire between coil and fuse, open in wire between coil and ground, short or open in wire between PCM and coil, bad PCM, open in wire between injector and PGM-FI main relay, short or open in wire between PCM and injector.

 

Update: i took the intake manifold off to clean the egr plate and it was almost completely clogged. Cleaned it and replaced all intake gaskets. After a brutal test drive the only code i have is p0420. So im assuming by the amount of carbon build up in the egr system that the cat must be clogged also.

 

Ok here's what's been done so far...
new plugs
New coils
Checked for vacuum leaks
Removed intake and cleaned egr system
Replaced intake gasket
Removed clogged cat
Checked compression 180 across the board

Still have multiple mis fire codes, the only thing i can think left to try would be fuel delivery.

Any other ideas would be greatly appreciated!