Toyota Repair: check engine light, fuel pressure regulator, egr system

Question
96 toyota Previa SC 133,000 miles.  The first code was po401.  I replaced the egr the vsv which tested bad it showed 0 resistance. I also replaced the gas temperature sensor since I couldn't test it and the same code returned after the repair. After 80 miles and all three of the above components were replaced.  The check engine returned.  The new code was bank 1 PO420.  The Po401 was also back.  My manual suggest a posible mechanical problem such as a vacuum leak or fuel pessure regulator either rich or lean.  The van idles fine and dosen't hesitate or stumble at high way speeds so I'm thinking I don't have a vacuum leak.  What should I chech next.  Can I trouble shoot the fuel pressure regulator.  I haven't noticed black smoke on start up,but the exhuast pipe looks sooty to me.  Any Help would be appreciated.  Frank

Answer
You need to take care of the PO401(insufficient exhaust gas recirculation(temperature) first because it can trigger the 420 code which is a catalityc converter code.
Try replacing the egr vacuum modulator valve, it's on top of the egr valve, check the vaccum ports on the throttle body make sure they are not clogged, check for proper vacuum readings at the ports that are connected to the modulator valve.


find someone who knows how to check the egr system for proper operation, it requires a vaccum pump and vacuum gauge and somenoe who knows how this system works, I would recommend the toyota dealer, again, fix the 401 code and the 420 may disappear, the 420 could also be caused by a bad oxygen sensor.