Has my cat cracked?

Driving around this morning just before I arrived home I heard a strange metallic crack noise but afterwards didn't feel any difference in the way the car drove and thought nothing of it. Started the car up a couple hours later and had a engine light on the dash and "check emission" message.

Scanned and got P0420 code - catalytic converter system bank 1 - efficiency below threshold

Monitored both pre and post lambda sensor voltage and once warm they both pretty much followed each other.

I guess it's looking likely that the cat has failed/cracked? Can anyone confirm before I try to get hold of one.

My pre-cat (on the manifold) is already gutted I believe so I'd be looking for a secondary one if anyone has a oem exhaust in the North East/North Yorkshire area?

Secondary cat hasn’t cracked hmmm, confirmed the primary cat is gutted while I was in there

I thought that aswell and I’ve had a good look around while it was on axle stands but can’t see anything. I’d expect even if the manifold had cracked that the post cat O2 sensor would still be lower than pre cat