What are codes p0401 and p0300 mean on a 2000 isuzu trooper

Many states will not pass a vehicle with a check engine light.

The P03000 code you have has little to do with the gas cap, rather it is a random misfire code. read more about this code and possible fixes in the link below. Good luck!

http://www.obd-codes.com/p0300

Maybe this is a PDF can help you :Diagnostic Trouble Code Table

PO300 code means miss fire on the cylinders, That is usually caused buy bad spark plugs end wires,PO401 code means a problem with the EGR ,Valve Sticking.

That means there is a big problem whenever it flashes. You need to get it in and get the code asap. When it flashes, they call that a major issue. Be careful and go asap. Autozone checks for free but you probably need a mechanic.

I had the same thing. It is something to do with emissions apparently. I was told to check the fuel cap ! I put some silicone on the seal, cleaned it up, put it back on with the correct number of clicks and after about 15 minutes of driving, the Check Engine light stayed off.

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.

your computer has a code

The 1990 Isuzu Trooper with a 2.8L GM engine, the diagnostic connector is located directly beneath the coin try in the front of your center console!!
Along with the typical OBD connector is two wires with a spade connector on each, one female and one male. Connect the two wires to display the diagnostic codes on your check engine lamp on the instrument cluster panel.
As for finding out your codes meanings, go to your nearest public library and locate a 1990 Trooper service manual.
 

I also had the same problem with my 2000 trooper.... the problem is u are low on fluid..... the reason u are low because your tranmission is leaking ..... it doesn't take must

yes they have 2 cataletic convertors, one each bank
there is EGR codes, but only front O2 sensor codes and fuel trim codes no CAT codes
what were the scanner codes or flash codes i can work with both for helpful info
you can read codes by bridging pins 5 & 6 with ignition in on position engine not running