I keep gettinga P0430 code on my 1998 GMC Sierra. It suggests a large vacuum leak, a running condition, or poor catalyst function. I switched the secondary O2 sensors side-to-side, and the same code came back on about 150 miles later, so the problem is not a bad O2 sensor.
Prior to the code first coming on, I changed the intake manifold gasket. I dont' think we missed anything (except for a fuel line that is seeping; we are going to tighten that down tonight). Is anyone aware of anything that can be causing this code? Should I just replace the catalytic converter?
That model year is notorious for intake manifold gaskets failing. I just had mine replaced, and the truck runs better than ever now. Are you sure there were no cracks in the plastic part of the manifold? Also, there is a revised gasket for that model. Fel-Pro makes it, and they call it the "Problem Solver" gasket. I would double check the intake manifold and the gaskets for the throttle body mount.
I used the Fel-Pro gasket set. I never removed the throttle body from the intake manifold; I removed it in one big piece.
When I switched out the intake manifold gasket, we failed to mark the distributor very well, and did not get it back into place correctly. Not that it mattered, since I broke the plastic where the distributor cap hold-down screws go, and had to replace the whole distributor anyway. Long story short, the timing was off for about 100 miles before I could get it fixed properly. The timing was far enough off that the CEL came on for bad timing. Would that be enough to dump gas through the exhaust and plug up the cat?
I just don't want to replace the cat and find out I have to do it again because I fixed the symptom, not the problem.