I have a 1999 Chevy Blazer, the check engine light (p0420 - catalyst efficiency below threshold) came on about 6 months ago. A mechanic told me not to worry about it. This morning the engine starting missing and a new code appeared (p0303 - cylinder number 3 misfire). I replaced the plugs and wires but this did not fix the problem. Is this all due to the catalytic converter?