O2 Sensor - OBD code 0138

Hey guys, 96 4.0 XJ 80k miles, been doing some maintenance and decided to see what the check engine light was trying to tell me the other week.

Seems that its giving a P0138, which I believe is the sensor behind the Cat. What I am wondering is this an indication that the Cat is toast, or just that the sensor is failing?

How would I know if the Cat is not doing its thing?

I know that I have a cracked exhaust manifold, which may have gotten worse since its sounding a little like a diesel right now. This is all following an episode where I didnt notice my coolant was low and the engine got hot 230ish (with steam). After inspection, I see that the water pump is leaking. So I have to change the water pump & rip out the cooling components for an overhaul. I cant put my finger on the cause of this new diesel engine sound, could be the water pump making a racket, could be the crack in the exhaust got bigger when it got hot, I dunno yet (Im in AZ and its like 130+ degree's in my garage so Im being lazy & driving the wifes 97 XJ)..

Is there anything I can detect when the Cat goes bad?

Thanks.

i believe the convertor and "possibly" anything emissions related before the convertor is 8 years-80000 mile federal warranty. is it over 80,000 and is the original delivery date less than 8 years?

I believe it is past all that.

I read around a bit more, but dont really have anymore ideas. I banged on the CAT and it sounds solid, no rattling. Visual inspection shows no obvious problems.

Oh well, I'll leave it for later. Im going to replace the cooling system and see where I am after that. New radiator, water pump, thermostat and hoses should take care of the cooling issues.

I am being tempted by the GDI 3 core that radiator.com sells, since its $130 and I can pick it up in town here. I've been searching for why some people dont like them and some do, but havent come across any definite issues other than people saying that the core tubing is smaller than say Modine or CSR.

Im comparing it to my stock one, and it seems the GDI is going to be alot better so I think I can live with knowing its going to be better than what I have now.