Missfire

You are not going to be able to do much until you replace the catalytic converter. They are monitored for proper operation, and a diagnostic fault code is going to be set, and the Check Engine light will be turned on. That will cause a lot of other self-tests to be suspended. Without the results of those tests, there is almost no chance of knowing which circuit or system needs further diagnosis.

The flashing Check Engine light indicates the most serious of problems. Too much un-burnt fuel is going into the exhaust system where it will burn in the catalytic converters and overheat them. That is typically the result of spark-related misfires. A good place to start is by switching the ignition coils between cylinders that have a misfire code, and some that do not. Erase the fault codes, then see if misfire codes set for cylinder you moved the suspect coils to. You can do that with injectors too.

If you have a metal tube going to each cylinder, for the EGR system, those can cause misfire codes too. The tubes become plugged with carbon. The last one or two cylinders with clear tubes get all the EGR flow and have the misfires. The cylinders with plugged tubes get only fresh air, so they fire just fine.

Gee, I don't know what's so hard to figure out. Just because I had the wrong ignition system in mind and told you the wrong thing, ... But I'd never admit that!

What I should have suggested was to replace the spark plugs and wires first, as you already did, then, in the absence of further test results, a less-expensive route is to substitute a new ignition coil pack. These cause enough trouble that it won't hurt to put the new one on a shelf for the future if it turns out you don't need it now.

You're going to have the Check Engine light on and the fault code will be "catalytic converter efficiency" for "bank 1" or bank 2", but the misfire codes will no longer set if the coil pack solves the problem. The engine should run okay with the missing converter. Don't forget to erase the fault codes, otherwise you'll be confused by the old misfire codes that remained in memory.