Catconverter burn-up..

[fixkick]

lets start over, who said the burned up 2 Catconverters now. by what measure, P0421 codes, or smog man or?

The P0400 means the EGR fails.....  in some way , if the cat broke up in pieces and went byebye EGR then fails too.  the cat must be there and must have normal back pressure or the EGR MODulatopr FAILs to work. (goes offline)

did you fail a smog test or something, please advice  here.
thanks for posting !

first one was obvious.  header turned dark from over-heat, lost all power slowly and dramatically, pulled the cat and it was melted to a rock.  I'm getting same subtle indicators now again. mostly progressive vibration from the cat as it sounds audibly to be deteriorating.  I'm in Minnesota so no smog test. 

no P0421, only P0400 that is incessant.  I clear the code, drive for a bit comes back, but no clear circumstance.   Sometimes first drive, sometimes second.  sometimes on first start up, sometimes at random while driving.  

New EGR, tested ISC good.  decent gas milage so not in limp, ego exhaust port should be open as the engine/header only has about 8000 miles on it after total rebuild.

only other bit of information I left out of original post is recently it has started to hiccup a bit.  at cold idle worst, or low throttle, not with open throttle.  not a full misfire, seemed almost like it had bad gasoline, but I'm on tank 3 from different locations and same issue.

fuel pressure is stable and good.

the first cat did burn up,  from engine super rich, that is what cause burn up , the CEL lamp also flashes when the ECU knows it is rich, the flash means ( go  slow or cat burns up)
the 2nd cat IDK,  why , same reasons? who knows.
but lets talk 2 things here, first off there are 2 causes of rich, (gross rich and simple rules)
it is in limphome ,  for many reasons, even 02 front dead,
or the ECU is lost.
lets do lost,  (02 dead,front means injection rates are now lost,  no real controls only the MAF output, and is purposefully rich, so you dont burn up exhaust valves from too lean.

Injectors can leak, or fuel pressure way too high, FPR bad and causes this, or 02 sensors is bad or reads wrong (exh. leaks near it)

the only way to prove closed loop is to see the scan tool show CL, for closed loop ,noT OL open loop cruising. (and at idle hot)
or scan the 02 and plot it. it must swing and swing on a 0.45v center. 
the can be no exhaust leaks near the front 02 or it goes rich, AFR super rich, max.

if the new cat broke up EGR is not the worry, the CAT must be good to get EGR working, EGR uses that CAT to gauge full engine load, and run EGR flows based off of that.
if the CAT melts, (engine goes weak cant pump air) then EGR overflows wrong, due to huge CAT back pressure and causes even more power loss...
maybe the second cat got hit by road debris.  or vibrated too hard and broke up, had a jeep do that 3 times,  (made a mount for the cat to keep it from vibrating and was cure.

is there not a frame mount for the cat? on this car, IRRC.  I forget, but will  look that up in the AM.

Seen one china clone maf used that run way too high and the eCu could not trim it down so high and ran rich, full time. o2 could not correct this error.

mGP on my 96 5sp was 28mpg, that is possible. tad less in town.

if the O2 hunts at light cruise say 40 MPH, the ECU and 02 are ok,  it must hunt around 0.45volts just like the scope views show.  no stuck high ,low or offset.
this and short turn true at same time also tells me what is up,

do the EGR stall test, and live test yet? 1996, up, the live test is rear wheels off the ground and 1mph or faster, no 0mph,
per my gen2, EGR page. and FSM rules on same.
my guess is the stall test passes, (exh paths all good)  and is vacuum issue, CAT blown out is top of list. for EGR death.