2004 Silverado check engine light

[Mark L Anderson]

I have a 2004 Chevy Silverado with a 5.3 I have an occasional check engine light that comes on. The code doesn't get stored in the computer but I got it checked when it was running and it comes up as having a problem with the catalytic converter or a miss fire. I have had the plugs and wires changed. It feels like it is still miss firing at times. Other times when you step on the gas it feels like it's lagging. Any ideas where to start?

[sefiroxx]

You'll need a scanner to see which cylinder.

Run the scanner while driving to monitor o2, fuel trim, sensors etc