Catconverter burn-up..

[fixkick]

Hey All-

1996 X90, 16v G16b.  No Engine mods

I've had my x90 back running since early this spring and have burnt up 2 Catconverters now.  I've done all the EGR tests with success, no limp mode, but consistent P0400.  I'm leaning towards bad oxygen sensors but I wanted to check in here and get alternate insights before dumping $$$ into both sensors and another new Cat.  

Scanner says I'm getting OxySensor output voltage, but shows "FAIL" on these scanner tests:
Sensor time; [rich-lean] & [lean-rich] - Fail
Minimum sensor voltage - Fail
Maximum sensor voltage - Fail
Time transitions - Fail

Yes, shows failures so its seems like a no-brainer, but I don't know where the scanner is retrieving the tolerances for those tests and therefor don't know how reliable those results are, so it seems intuitive to inquire here also.

Here's what the scanner is showing generally at warm idle [783rpm average]

Fuel System1: CL
Calculated Load: 17.6%
Coolant Temp: 172f
Short Term Fuel Trim - Bank1: -0.8%
Long Term Fuel Trim - Bank1: 3.9%
Intake Manifold Absolute Pressure: 8.1 inHg
Ignition Timing Advance: 9.0 degrees
AirFlowRate from MAF: 0.3 lb/min
Oxy Sensor Output Voltage B1-S1: 0.740V
Oxy Sensor Output Voltage B1-S2: 0.545V
Short Term Fuel Trim - B1-S1: -0.8%
Short Term Fuel Trim - B1-S2: 99.2%


Any thoughts or insight on the subject would be very much appreciated!

the front sensor must work,  S1
and if not that causes limphome, and rich AFR,
the sensors has 2 parts, heater and the O2 cell.   both must work.(inside)
if you had  scan tool that would graph the o2 sensor out output, it must swing like , hunting for perfect air fuel rato (AFR)
like this.

blue trace bottom


and this hand held tool




more here.

https://fixkick.com/ECU/Authority/A_Full_OBD2_scan.html