I have a 2000 neon, my friend said when he bought it the guy said the convertor was clogged. I cut it open and it was not. I removed the inner and welded it back up just to see if it would run. It did run but supper high idle. I replaced the MAP sensor and it idled down. Only problem is it is rough. I have a current code of 304 a missfire on number 4 cylinder? i can remove battery and it clears it but back in a few and the idle never changes,it never dies just idles rough. when you give it gas it runs real good , pretty fast for a four cylinder. It smells rich too, you can smell fuel in exhaust. Any ideas?

 

bad plug or wire to cylinder #4? try swapping number 1 and 4 wires around and see if missfire code changes cylinders too, also try swapping plugs

there is gonna be fuel smell at tailpipe if its missing(everytime that cylinder doesnt spark it sends raw unburnt fuel down the tailpipe)

so let me get this straight you removed all the innards of the CAT and welded the case back together? if thats correct good luck passing emission testing

 

If you gutted the cat, that will make the exhaust smell like gas quite a bit more too.

 

Yes I removed it and welded it back. We do not have a yearly test to get plates. Ihave replaced the plugs and wires.It is a autozone set but should still be ok for this application I would think. I will try flipping to see if the code changes. I wondered if the cat was not right would that cause the O2 sensor to read funny and run rough?

 

usually to test a plugged cat we just put a pressure gauge in where the O2 sensor goes. excessive backpressure would indicate a plugged exhaust.

 

Check the coil pack where you plug the wire onto, it may be corroded.