SOURCE: Oxygen Sensor on '99 Isuzu Trooper
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.
SOURCE: where is the egr valve for a 1999 isuzu trooper?
The EGR valve is a cylindrical unit above the engine. It would be behind the
throttle body.
It will have a cable connected to it. Unplug the cable.
There two bolts. Simply remove.
An that is all there is to it.
Simply put the new EGR valve in its place, screw in the bolts and reconnect
the cable.
Make sure to see a gasket. The gasket can stick to either the EGR valve or
the throttle body. It does come off easily but since it is metallic, it looks
like it part of the throttle/egr valve. 
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SOURCE: where is the location of the egr valve on a 2001
on petrol V6 engine its right rear side of inlet manifold(left side when looking into engine bay) see pics. below
diesel its on inlet manifold left side middle section of manifold
SOURCE: How to clean EGR valve passages in 1993 Isuzu Trooper DOHC V6
You have to the EGR valve (sometimes the Intake manifold to EGR Vlalve line also). After you remove it take a drill bit the about 20 % smaller than the actual hole of the outlet side and rotate the drill bit with your hand to dig out the carbon. Aditionally the line coming from the intake manifold to the EGR valve will clog near the connection to the EGR valve. I use either a single piece of copper wire or the wire that you would use to tie ribar together to clean the line. When your done cleaning take a piece of vacuum line and connect it to the diaphram side and **** on it... then plug the line well with your thumb. Grasp the valve and blow thru it... Air should blow thru it freely. Remove your thumb from the vacuum line and try blowing again.. No air should pass thru the valve.
Then reinstall the valve and start the vehicle with the vacuum line disconnected (but blocked off) Use another piece of vacuum line connected to the diaphram again and **** on the line.. The vehicle should die or run really bad. If it does this then you cleaned it correctly.
SOURCE: 2000 isuzu trooper: OBD-II Trouble Code: P0401 Insufficient EGR F
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.
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