PO401 EGR Valve (Exhaust Gas Recirculation)
The EGR valve is bolted to the upper intake on the drivers side near the back. The top looks like a UFO. Problems here will reduce gas mileage and cause pinging.
The PCM (powertrain Control Module) calculates how much EGR is needed as you drive. It sends electric pulses to the EVR (EGR Vacuum Regulator) which opens allowing a controlled ammount of engine vacuum to go to the EGR valve. The vacuum pulls the EGR valve open allowing exhaust gas to pass from the exhaust manifold, through the EGR tube, to the intake manifold and on into the engine. As the gasses pass through the EGR tube they go through an orifice. Two smaller hoses connect from both sides of the orifice to the DPFE sensor (Differential Pressure Feedback EGR). The DPFE reports back to the PCM the ammount of EGR gasses passing through the tube so that the flow can be controlled. This is a closed feedback system.
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Often as the EGR gasses cool down they deposit soot which eventually blocks the system. This usually happens right behind the Throttle Body. Only solution there, is to remove the TB and scrape the deposits out.
The EVR (black plastic device just to the drivers side of the coil pack) can leak when it gets old. If so it must be replaced.
The DPFE (way down on the back of the engine) can also fail.
The vacuum hoses and wiring to the sensors can also fail.
Blocked EGR passages are very common and the best place to start. Do a search for various ideas at testing or repair.
As for the catalytic converter, a back-pressure test is the only test you are likely to be able to do at home.
Be sure to read this You probably have some other issues also. Be sure to read the FAQ's on the IAC, IMRC, and cleaning the MAF.