P0430 & P0420 code

[LemonySoda60432]

Good afternoon. I have a 1999 GMC C1500 with a 350 Vortec. Bone stock unless the previous owners messed with her. I have slowly been restoring the truck and it is my daily. My first car too, as a matter of fact. I have one problem though. When I bought her she had no check engine light, after a few months she flung a downstream o2 sensor code if memory serves. I replaced that sensor with a GM genuine part and reset the light. After driving for a while, she popped a light again so I pulled into autozone to get it scanned. Now I am getting both P0430 & P0420! With their little printout saying the most likely culprit(s) are the Fuel injectors and/or the intake manifold gasket. Problem is I don't think it could be the manifold gasket because she doesn't idle high. Any ideas on what it could be? Thanks.

 

[df2x4]

P0420 and P0430 are both low efficiency codes for catalytic converters unless I'm mistaken, bank one and bank two. If you have no other codes I'm inclined to think your catalytic converters are both not working properly.

 

[Dariusz Salomon]

I agree with df2x4-what may be happening is what happened to me( and happening again) with catalytic converters-they disintegrate inside. I replaced one( spliced in) and now the other one is rattling.

 

[LemonySoda60432]

P0420 and P0430 are both low efficiency codes for catalytic converters unless I'm mistaken, bank one and bank two. If you have no other codes I'm inclined to think your catalytic converters are both not working properly.

Thanks for the swift reply! Another tidbit of info, my old sensor smells slightly of gasoline, is that normal or could my fuel injectors be dumping too much fuel in the engine or is it a sign of bad cats?

 

[df2x4]

Thanks for the swift reply! Another tidbit of info, my old sensor smells slightly of gasoline, is that normal or could my fuel injectors be dumping too much fuel in the engine or is it a sign of bad cats?

That I'm not completely sure about, but the fact that there are no other codes present and the truck is running normally makes me suspect that everything except the cats is probably working properly. I'd still want to be sure before replacing the cats though, as they're expensive and in my experience the aftermarket options available aren't nearly as durable/reliable as the factory ones.

There is a diagnostic chart for both of these codes in the '99 GM factory service manual, which is available for download here free of charge. Page nine of the stickied Service Manual thread in this section, post number 77 by PM18S4. This link should take you right to the post.

https://www.gmt400.com/threads/88-98-service-manuals.43575/post-1234512

The P0420 diagnostic chart is on page 6-1192 in Volume 2 of the manuals, but explanation of the code starts a few pages earlier. As mentioned there, I'd be checking for exhaust leaks.