My 2000 Isuzu Trooper needs a catalytic converter.

P0401 is EGR problem
P0300 is a cylinder 3 misfire.
Replace EGR valve and do a full tune-up.

The most probable cause would be blocked system. However code p0420 refers to catalytic converter system bank 1--efficiency below threshold===causes--catalytic converter faulty---wiring ---HOS2. It is more likely that the oxygen sensor needs attention and the cat converter needs replacing

I don't think it is your catalytic converter because your light usually flashes and a bad cat I never heard of them causing a misfire. I ran your other code it is for a faulty EGR system. Exhaust gas recirculation excessive. You are getting too much exhaust gas back in the system. This will cause your misfire. Go-to a trusted mechanic and have them check out your EGR system.

Yes and No. The EGR can cause the car to idle rough and or stall. I am assumeing the check engine light was on and set code for EGR and Catalytic Converter. If the engine isn't running right it is possible the cat won't work right due to improper air fuel ratio that the EGR is causing. To do repair proper the EGR should be fixed then clear codes and see if the code for Catalytic Converter. There are other reasons that would your car to idle rough or stall I am just assuming that this mechanic got codes or tested the EGR right.

Your SUV is really giving you a hard time, lol. Sorry but I've had those days too.
I would try to check if you have blocked EGR passages. If ok check for restricted MAP sensor vacuum hose/passage or restricted exhaust. I know you replaced the CAT but if the muffler is blocked then there would still be a restriction. Also check the wiring and connection to the MAP sensor.

what is the code that turned the light back on?????? if its a P0420, the cat is not good enough. you may need one good enough for California or a better quality converter.

clogged , check the rest of your exhaust after cat to see if it gets hot

you can buy a no contact thermometer for less than $55.00

but the shop that installed the cat should have that

good luck

Without knowing the specific code, I will assume it relates to an inefficient catalytic converter. At this mileage it could very well mean that the catalytic converter is failing to do it's job and needs to be replaced. The computer sets this code when it sees a signal from the rear O2 sensor start to change in time with the front O2 sensor. It should not. If the cat is working, The rear O2 sensor should have a stable output as the cat is supposed to be removing all the oxygen from the exhaust. Sometimes this code will set if the spark plugs or the engine's state of tune has deteriorated and is producing more emission than the cat can handle, the rear O2 sees the overflow and assumes the cat is bad.
At your present mileage, I would strongly suggest changing the spark plugs if they have not been done in the last 20,000 mi. Have the light reset and drive it to see if the code returns. If it does, replace the catalytic converter. I suspect that you do need a cat, but I view the spark plugs as maintenance, and I wouldn't want to put a brand new cat in without the assurance that my old plugs were not going to damage the new one.
Hope this helps,
Dave

There are many reasons for a PO171 code, these need to be checked before replacing O2's:

Vehicle driven low on fuel or until it ran out of fuel

One or more injectors restricted or pressure regulator has failed

Fuel delivery system supplying too much or too little fuel during cruise or idle periods (e.g., faulty fuel pump, or dirty fuel filter)

Air leaks after the MAF sensor, or air leaks in the PCV system

Air leaks at the EGR gasket, or at the EGR valve diaphragm

Exhaust leaks before or near where the front HO2S is mounted

HO2S is contaminated, deteriorated or it has failed

Fuel control sensor is out of calibration (i.e., ECT, IAT or MAP)

Base engine "mechanical" fault affecting one or more cylinders.

The O2 sensor is on the engine side (up stream) before the catalytic converter in the exhaust pipe, it is barrel shaped about the size of a jumbo magic marker.