What causes a catalytic converter to fail?

[dave88sw]

Shortly after i bought my Discovery V8 (about 5 years ago), the cats started to rattle and eventually broke up completely, filling the centre silencer with lumps of cat.  I wasn't particularly happy with the way it ran at the time and after a bit of poking about found the air flow meter was dead amongst other things so it was running very rich.  I put the death of the cats down to that.

I fitted a second hand pair (along with a new stainless cat back system) that appeared to be in good condition and had no trouble passing the MOT, emissions were very good (I have fitted megajolt with the original 14cux so it runs really very well) and ever since, i've been happy with the way it runs and recently got over 18mpg on a trip to cornwall. All this leads me to believe it's running as it should be and the mixture can't be too far off, however, the cats have just started rattling fairly loudly at idle from time to time. 

What causes the cats to fail?  Is it simply because i bought a second hand pair, so they've probably done as much as the originals?  I'm happy enough to spend the money on some good quality replacements (any recommendations?!) but only if i'm confident that they'll last and it's not going to kill them too.

 

Thanks

Dave

[lo-fi]

Running really rich tends to make them overheat, which can break up the substrate. Burning oil similarly, and also coolant getting into the exhaust can have an effect. A knackered plug or lead can cause a cylinder not to fire efficiently, sending some unburned fuel down the exhaust, which is not always obvious on the V8.

It could equally be that they're just old, been abused in a previous life and had their day now.