Check engine codes

https://www.2carpros.com/trouble_codes/obd2/p0400

P0420 - Catalyst System Efficiency Below Threshold (Bank 1).

That generally means the catalytic converter needs to be replaced, but an engine performance specialist would typically performs some tests first to verify that.

Bank 1 is driver's side. The front oxygen sensor detects unburned oxygen in the exhaust, and switches between "rich" and "lean" about two times per second. If the catalytic converter is working properly, the oxygen sensor after it will switch perhaps once every minute or two. If no change in the exhaust gas takes place inside the converter, the rear oxygen sensor would switch at exactly the same rate as the front sensor. As the converter loses its effectiveness, less and less change takes place in the exhaust gas, and the rear sensor switches faster and faster. That is how the engine computer knows when the catalytic converter is not working, and it sets the "efficiency" fault code.

Drivers side on a GM 5.7. If yours is OEM it will be the front catalytic converter of the two.
If you are not in California or New York you can get the Walker 50410 unit that replaces both for around $300.00.

No. Different codes are set for sensor problems. Code 420 is set when that problem is detected with properly-working sensors. If either sensor was bad, the converter's inefficiency could not be detected, and code 420 would not set.

Fuel/air mixture is not affected by engine speed.

A plugged catalytic converter will cause a severe loss of power and usually leads to it overheating. There are two other problems. First, since this affects emissions, the check engine light will be on all the time. If another problem occurs, you will never know it. Many minor problems can turn into expensive repairs if they are ignored. Second, there is always a long list of conditions that must be met for a fault code to be set, and one of those conditions is that certain other codes are not already set. When a code is set related to something that is needed for reference, the tests that need that information for comparison are suspended. When those tests are not run, defects will go undetected. You can have a new running problem, and no fault code to tell you where to start looking for the cause. Once the first problem is repaired, the other tests resume, then other defects are detected, new fault codes are set, and the check engine light turns right back on again.

Whether you replace it with the pipes or not depends on price, ease of access, and how the system is assembled. You will have to look at it to see if it is welded. Back in the 1970's GM catalytic converters were bolted on and were easy to replace, but those flanges caused more trouble than the converters did. They often rusted to the point they would pull off and fail to hold the pipes tight enough to seal. Systems that are welded need to be cut apart. If the replacement comes with more pipe than was removed, you may be able to expand one and slide it over another, then use a standard exhaust pipe clamp to hold it on. If the new converter is just the body with no pipes, you have to be sure to cut the old one out and leave enough pipe to reach, then it will have to be welded back together. Only a wire-feed welder works for that. A stick-type arc welder will burn through the thin metal. Brazing is not strong enough unless two mating pipes slide into each other first. The body is made of stainless steel, and the pipes it comes with usually are too. That is harder to crush with a clamp than is regular steel exhaust pipe. Stainless steel is harder to weld too.

If you replace the converter with a straight pipe, you will have what you have now; an ineffective converter that does not do anything, and the check engine light is on. Both oxygen sensors will "see" the same thing, so the computer will know no change is taking place in the make-up of the gas. Replacing the converter with a pipe is a real lot of work to end up with the same thing you have now.

Be aware too we read a lot here about people having repeat or new problems when using universal aftermarket catalytic converters. Their efficiency was not monitored on 1995 and older cars, so you never knew if they were working properly. Anything will work on those cars. A lot of shops will not replace exhaust parts with anything other than what comes through the dealer, especially if they are in an area that has emissions testing. A universal converter can work fine on one car model, but when used on a different model, it may not pass the emissions testing.

Oxygen sensors don't work until they get up to 600 degrees. The Engine Computer knows that so it won't run any tests on that part of the system at that time. There are other conditions that can cause some tests to be suspended. There may have been something else wrong that prevented the computer from running some tests, or something that caused it be unable to believe the results. Many things have to fall onto place for a fault code to set.