Not that I mind either way but I thought it was agreed back in January that this thread should be about ways of preventing the cat issue rather than a place for discussing what causes the cat issue?
If we're talking what causes the cat issue we should bear in mind that Nissan recalled Elgrands in Japan (where they all obviously run on Japan fuel which some on forum reckon is superior to UK spec fuel).
Nissan blamed the problem on a fuelling issue, which a recall was supposed to address. I do know they go richer than most vehicles at full load (foot to the floor type situations) even when the recall has been done, and I do know that if the recall hasn't been done they continue to inject a little petrol (too lean a mixture to burn) during over-run situations.. (I can see all this in diagnostics readings from either NDS2 or from LPG software while running on petrol) the recall stopped the little petrol during over-run situations. I don't see the fuelling recall making any difference to longevity of cats.
If we're going to blame the cat problem on cats getting hot we should remember that cat temperature is related not only to the temperature of exhaust gasses coming out of the engine, cats produce a lot of heat themselves when they are doing their emissions clean-up work... chemical reactions take place inside the cat (which is the whole point of the cats), the chemical reactions are exo-thermic (produce heat and lots of it). Exhaust gas temperature may be hotter with a lean mixture than with a correct mixture or rich mixture but still cat temp may be a lot hotter with a rich mixture than with a lean mixture, because of the chemical reactions inside the cat that the rich mixture entails.
Just about all major emissions that a cat tries to clean up (HC and CO) are lower on LPG than on petrol, other emissions such as those from burning sulphur etc are negligible on LPG compared to petrol. Lower emissions from the engine means less work for the cats to do (less work as in less chemical reactions which means less heat produced by the cat). This is true provided other things are the same, such as mixture. During normal driving conditions we can presume mixture is correct if the engine warning light isn't on. A good LPG installer will set the high load mixture leaner on LPG than on petrol... On petrol there can be a slight power gain from running an overly rich mixture, on LPG maximum power will come from a mixture that while still slightly rich isn't as rich as the overly rich petrol mixture. The leaner mixture at high engine loads (foot down) will mean less work for the cats to do.
An LPG installer can prevent the engine getting an LPG mixture that is too lean to burn during over-run conditions regardless of whether the fuelling recall was done or not and can make the top end mixture leaner on LPG than on petrol regardless of whether the fuelling recall was done or not.
Another thing that will cause cats to heat up is if there is a difference between mixture on different engine cylinders on the same cylinder bank. The oxygen sensors in the exhaust measure spare (unburned) oxygen content and give a signal that corresponds closely to average mixture for 3 cylinders (erring on the side of the leaner running cylinders). If one or more of those cylinders runs a little leaner than others it means others are running a bit richer (and vice/versa) because of 'average'. Cats combine HC + O to produce H2O and CO2 and combine CO + O to produce CO2. A cylinder running a bit lean will pump a higher than average (average being correct mixture) amount of O into cats, a cylinder running a bit rich will pump a higher than average amount of HC and CO into cats.... So if there's a mixture mismatch between cylinders the cats get more work to do which will make them hotter than if mixture was consistent across 3 cylinders. A gummed petrol or LPG injector can cause such inconsistent mixture.
Steve Thompson has recently passed MOT without any cats fitted (option 5) running on LPG but would have failed emissions running on petrol without any cats fitted.
https://forum.elgrandoc.uk/threads/lpg-mot-fail.22417/ There was a bit of drama along the way to passing the emissions test but that was because the tester tested it against the petrol emissions standard and didn't know it was running on LPG. If it wasn't for this slight drama there probably wouldn't have been a 'LPG mot fail' thread, it would be just another than had quietly passed MOT emissions running on LPG with no cats at all fitted like the many others before it and many others after it.
Others have mentioned metal rings in front cats being difficult to remove during front cat decore. When I recently decored front cats on one of my own Elgrands I found no metal ring inside the front cats. I didn't even have to cut front cats open to decore them (when removed from the vehicle I found I could decore them from one end)... but there is still plenty work involved removing front cats from the vehicle before front cat decore can be done, then they've got to be refitted. But the method I used means no parts have to be bought at all, the only costs are time/labour to decore all 4 cats.
https://forum.elgrandoc.uk/threads/front-cats-decore.21762/
I have converted around 300 Elgrands to LPG and own 2 myself. I'm converting one at the moment (I've also fitted a towbar on this one I'm LPG converting), I have another coming in to convert on Wednesday, another on Monday, another on Wednesday, etc etc. I tend to leave Fridays available to fix LPG installs that were fitted elsewhere (including on a lot of Elgrands, like the one coming this Friday). When I convert them I make sure the calibration/mixture are correct and make sure they run a little leaner at the top end (under full load enrichment) than they do on petrol. IMO along with cleaner emissions from LPG anyway this should be kinder on cats than running on petrol.
If on the other hand an LPG installer does not set up high load mixture correctly, such as if they leave calibration to some sort of ECU self learning process, the high load mixture is likely to be much richer on LPG than on petrol and this could certainly contribute to cat failure because although exhaust gas temperatures will be cooler (having the rich mixture) the cat will have much more work to do which will see it get much hotter. Or at the other extreme (if the LPG install provides way too lean a mixture under high engine loads) burned engine valves. An LPG installer must manually set high load mixture, 'autocalibration' type features will usually see an engine of this type run too rich on LPG unless a proper job of manual calibration is done but there's also the possibility of too lean a mixture.
Simon