SES: Codes P0300, P0420, P0420PD

[Enrogersgm]

I just got an old farm truck this weekend and the SES light came on the day after I picked it up. I pulled the codes and three came up: P0300, P0420 and P0420PD. The truck is a 1997, K1500 4.3L with 120,000 miles on it. Any thoughts? One other thing that is strange is that I took it through a car wash before I drove it home and the truck stalled out in car wash (during the underbody wash cycle). It misfired like crazy for a good 15 minutes and then ran fine again.

Does the following make sense to clear the code?:
1. Replace plugs and wires
2. Replace O2 sensor

The truck has been sitting for good part of the last 2 years. I am 99.9% sure it has never had a tune-up. Is there anything else I should look at? I think it stalled out in the carwash because one of the plug wires is worn/exposed and it caused the truck to stall out. Also, it idles a little rough (not terrible) but is fine once it is moving. I did put fresh gas (and injector cleaner in it) so I think the issues are with the plugs/wires.

 

[jeff's88silverado]

I'd start with a full tune up plugs,wire,dist cap, rotor button

 

[Dubya]

I'd start with a full tune up plugs,wire,dist cap, rotor button

x2

p0300 engine misfire detected.
p0420 catalyst system efficiency below threshold. your cat converter is not working properly, o2 sensor not workin or exhaust leak damage pipes etc

 

[Enrogersgm]

Thanks for the help! I am putting in new plugs, wires, cap and O2 sensor tomorrow. Hopefully it takes care of the problem.

 

[Enrogersgm]

Well, I got a tuneup and the left bank O2 sensor replaced. The light stayed off for a day and is right back on. Any chance that is it still one of the other O2 sensors? There doesn't appear to be any visible damage to the catalytic convertor, or any part of the exhaust system that I can tell. Not sure what the problem is. A little annoying because I get good gas mileage and it runs fine. I need the light cleared so I can get it registered.

Thanks

 

[Enrogersgm]

I forgot to mention that on the P0420 code is showing up now...