Mil p0420

Could also be an O2 sensor. I looked up the code, these are the possible causes -

* Leaded fuel was used where unleaded was called for
* A damaged or failed oxygen sensor (HO2S)
* Downstream oxygen sensor (HO2S) wiring damaged or connected improperly
* The engine coolant temperature sensor is not working properly
* Damaged or leaking exhaust manifold / catalytic converter / muffler / exhaust pipe
* Retarded spark timing
* The oxygen sensors in front and behind the converter are reporting too similar of readings
* Leaking fuel injector or high fuel pressure
* Cylinder misfire
* Oil contamination

More info here -

P0420 Error Code: Catalyst System Efficiency Below Threshold (Bank 1)

Thanks!
leaded fuel not possible since it was banned since 1993
Downstream O2 sensor was replaced already.
O2 wiring connection checked, all O2 sensors signal voltage jumps between 0.1 and 1.1
ECT, just replaced this when I changed thermostat housing assy
Leaking exhaust, now this is something I ned to check. Exhaust sounds horrible and I can hear exhaust leak in the cat bank1 area. I checked and found out the cat has welds which looks like some one tried to cut it open and weld it back about 2x2 inches.

Just an update... still getting P0420
I have now converted to dual exhaust. Basically fabricated the driver's side pipe, deleted the y pipe and installed H pipe.
I managed to check for exhaust leaks and now leaks are fixed. But still getting P0420 after driving around 200 to 300 kms.

Is it advisable to get used cats from the junk yard?