98 2WD Suburban Computer Question

[shultzee13]

You guys ever seen the PCM go out on these trucks? I was having a code PO430 passenger side Cat code and I replaced the Cat today code gone.
I have been driving it for about 6 months with that code. That code was always there during those six months.
Now today I get the new Cat installed and bam....Code PO300/PO304 misfire cylinder 4.

This makes no sense to me. Fixing the Cat has something to do with this?

I can replace the spark plugs but I just put a whole new set on 20,000 miles ago.

What are the odds the computer is just throwing out random codes making me chase my tail?

Thanks

[b454rat]

Anything is possible but could be the spiders are getting dirty or the EGR dirty, mine would do same thing. Drive it and it’s fine, but all of a sudden the light would come on. Of course when I need to get it inspected..

[speedygonzales]

In your case, I did not look up which code is dominant however I will say this.

Sometimes when looking at codes, the manual will tell you to look for certain other codes FIRST.

In the case of a misfire, that can cause the failure of the cat. In 30 years of dealing with PCM's of one sort or another, I've never seen one fail. There are conditions that cause them to act irratic that has nothing to do with the PCM itself. Such as diodes going bad in the alternator. This allows too much AC voltage to be present on the supply line of the PCM causing all sorts of "brown" issues.

That having been said, your implied assumption that the spark plugs are the only cause of a misfire is not correct. It was stated the injectors could be the cause. So to could the wires, the distributor cap, rotor and not likely the coil. Even less likely is the valves.

Did you know the coil is considered a tune up item and should be changed periodically? Though it's not likely the cause of a single plug misfire.

First thing I would do (if you haven't already) is look at the cap and rotor. If that looks good, I would swap out the plug wire with another cylinder.