I sent you a question a while back and you really helped me out. So now I've got another doozy for you. I have a 1996 Chevy C-1500 Pickup with a 5.7L. I just recently had to replace the old CSFI unit with the upgraded MPI due to a misfiring problem that wouldn't go away.
After the upgrade it ran great for about a week, and then I got another SES light. I had it checked out at Auto Zone and they said that it was a Cat Converter failure in Bank 1, possibly caused by a large vacuum leak or bad O2 sensor. When looking I noticed a very strong gas odor coming from the engine and that the EGR valve had a line that looked like it had been sheared at the elbow and now goes to nowhere. It doesn't seem to be pulling any vacuum, and I can't find where it goes to.
So I'd need to know if there are any diagrams available and if this could be the problem at all, or if I should look elsewhere.
It sounds like Trouble Code P0420: TWC (three way cat) System - Low Efficiency Bank 1. Regardless of the MFI conversion, you still want to cover the basics. There are faults other than the catalytic converter itself which can cause PO420, and typically there will be additional trouble codes present (O2 sensor or misfire related) which should be diagnosed first, before condemning the cat.
There's an O2 sensor located before and after the catalytic converter. The two sensors determine the catalytic converter's efficiency by measuring oxygen content before and after the exhaust gasses pass through the converter.
The smell of fuel and a vacuum fitting disconnected (EGR) are red flags and should be dealt with before anything else. If you smell raw gas under the hood, there's a fuel leak that's gotta be fixed first. A service manual will provide a vacuum diagram to help determine if that open port is actually an issue. You might want to check with ALLDATA. ALLDATA offers vehicle-specific online service information at monthly and yearly rates.
After checking the basics, bang on the catalytic converter and listen for anything rattling around. That's a sign that the converter is coming apart inside and can no longer do its job. You also want to look for any physical damage or exhaust leaks anywhere in the exhaust system. If you're running out of options get it to a dealership or a good independent shop, if for nothing else, an accurate diagnosis.