Chevrolet Silverado Misfire Reduced Power and Catalytic Converter

Question:
I have a 2004 Chevy Silverado which i just purchased today. I was on the highway driving and pulled off into a mall and my check engine light came on and the truck started missfiring and sputtering like it was going to stall. In the odometer window it said engine under powered and it wouldn't go over 2000 rpm or 30 km/hour. After shutting the truck off and restarting the truck ran fine but don't want to leave it cause I have heard it can get real costly real fast if the oxygen sensors are gone. 2004 Chevy Silverado. Engine size = 5.3L v8. Warning lights: engine under powered (in odometer window). I shut off the truck and the problem went away but the check engine light stayed on and my dad said he believes it may be the oxygen sensors and if it isn't fixed asap it could wreck the catalytic converter.

Answer:
It probably is not the oxygen sensor causing your issue. There is an electronically controlled throttle body on your truck that will usually cause this reduced power problem when it goes bad. It usually puts up a message like 'engine power reduced', not the 'engine under powered' like you state.

Either way, it set a trouble code and that is why the check engine light is on. Be sure to check the spark plugs for a misfire. You would need to get it checked out by a shop or someone with an appropriate scanner. If it has any throttle body codes, or TAC actuator codes, or throttle position, correlation, etc. You will need to replace the throttle body and throttle body gasket. The idle will then need to be relearned after replacement or the engine RPM will surge up and down.

Question: 2000 Chevy K2500 5.7
I have a 2000 K2500 with 174,000 miles. The truck runs a bit rough in damp wet weather until the engine is well warmed up. I have had the truck tuned up a couple of times over the years. Last tuneup about 30,000 ago. I am not getting any trouble codes but it feels like a missfire. However the spark plugs appear to be in good shape. New PCV Valve, Air filter, 02 Sensors are original as is TPS switch. The truck is getting 10mpg at best around town. What should I do short of buying as new truck. I would like to fix truck myself instead of taking to a mechanic.

You could be getting a slight misfire from a bad ignition coil assembly. They can arc around the metal case or bracket, especially when it is damp outside. A bad coil wire can do the same thing, as can a distributor cap and rotor that has condensation inside it. This is a common problem. You would notice green corrosion under the cap. I'd definitely lean towards an ignition problem, since those components are the most affected by weather and cold start conditions.

You might want to do another full tune up, including cap, rotor, wires, coil, plugs, etc. Also look for any vacuum leaks that may be causing a rough running condition. Sometimes intake gaskets suck air. They ere reduesigned years ago to prevent this on a cold engine. Also check the throttle body gaskets and for loose and rotted vacuum lines too. Since it has a lot of miles, concentrate on wear and tear, and maintenance items, and that should help your problem.