Long story short in my '97 C1500 with a 4.3L and a 5 speed needed a tune up last spring so I put all new plug wires cap and rotor on the motor and it ran like a champ. Also previous to that, I had the catalytic converter replaced in the winter as the original one was all broken up.

Truck has been running fine in the dry weather and just this last weekend the truck was misfiring pretty bad and set a SES light. I had it scanned and the only code that popped up was a P0420. Cat inefficiency code. Last night I put new Delco platinum plugs in. Truck ran a little smoother but still misfiring in the damp air. It was even worse with the defrost on (maybe the defrost advancing the timing made it worse?). The only thing I can think of is the cap and rotor is **** causing the engine to emit dirty spent exhaust into the converter and making the code set. I am waiting for my OEM Delco cap/rotor to come in. I was an idiiot and put a Wells brand cap/rotor on last spring. I always preach to you guys to use OEM ignition parts, now I'm eating crow. Hope this remedies it. Looks like I have to drive my '04 to work for the next couple of days.

 

Could the cat inefficiency be caused by shitty o2 sensor(s)? Just wonderin. Also, not tryin to be a dick, but are u sure plug are tight & all plug wires secure?

 

Yes sir! Everything is tidy and tight and routed properly.

 

I am going to dust off my old auto x-ray tonight and see if the 02 sensors are switching properly. The truck also misses under WOT. To the best of my knowledge oxygen sensors aren't used for WOT fueling. Am I right?

 

Can anyone tell me if the post cat 02 in the 97 trucks has any bearing on performance? Or is it like our trucks where it doesn't do a thing other than set the light?

 

i HAD A P0420 TRIP WHEN MY CONVERTER MELTED INSIDE CAUSING A RESTRICTION AND MISFIRE IN THE UPPER RPMS.

 

I have a hard time believing the converter is bad. It's only 3 months old or so. I am waiting for my new DELCO cap and rotor to show up. Until then, the truck is going to remain parked.

 

We see misfires cause cat inefficiency codes at work, my guess is the cat can't oxygenate the excess unburnt fuel and the post O2 picks up the rich condition and trips the code. There is usually a misfire code along with it also though. I've also seen clogged cats cause misfires, even though the cat is new, it could still be defective. We've had brand new cars have bad cats, it happens. Usually a clogged cat will only cause the misfire to be under load, in the upper RPMs though...if it's missing at idle I doubt that the cat is at fault.

I believe that the rear O2's work the same way as ours on a 97...just for cat monitoring, but I'm not 100% sure.