The front cats are the ones that break up, when they break up they cause ceramic debris. If rear cats are intact when front cats break up the debris invariably clogs the entrance point of the rear cats, so exhaust gas and debris can't escape, which causes pressure to build up in the exhaust and some debris to enter the engine where it acts like a grinding paste.
The front cats do more to clean up emissions than the rear cats do but the front cats are more difficult to access, so the old advice was to decore rear cats. With rear cats decored if front cats break up any debris that escapes the front cats can flow out of the exhaust (or at least flow into the middle exhaust silencer) thus allowing exhaust gas and debris to escape so there's much reduced chance of debris entering the engine. Having rear cats decored is much safer than having no cats decored but still isn't totally safe... Because front cats can break up in a way which sees them block themselves up, in which case decoring rear cats still won't allow exhaust gas and debris to escape and debris will still enter the engine and act as grinding paste.
So decoring rear cats is a good preventative step, far far better than doing nothing, but it doesn't completely rule out the possibility of a front cat still blocking itself up and still causing engine failure.... which has happened to more than a few owners. Decoring front cats (or perhaps replacing front cats with aftermarket cats that are not made of ceramic material) is the only way of completely preventing engine failure due to front cat failure.
Many owners who had rear cats decored but left front cats intact are unaware that since having their rear cats decored one or both of their front cats has since broken up. In these cases if front cats had failed before they had rear cats decored they would definitely be aware because of driveability problems (lack of engine power etc) until the blockage at the entrance to the rear cat had been manually cleared, but by the time they've realised what the problem was and cleared the exhaust the engine might have already been damaged - even a little ceramic inside the engine will wreak havoc on the softer engine metal. In these cases they're not usually aware that a front cat broke up because having had the rear cats decored, and such was good luck, the debris created went straight out of the exhaust instead of entering the engine... But having had rear cats decored and now that the front cat has effectively decored itself it'll probably fail the next MOT emissions test and the owner will be looking at the options (and spending money) again.
Many owners who decored front cats but left rear cats in place have passed MOT emissions tests but some have failed emissions tests. There is a reason catalytic convertors are always fitted close to a vehicle's engine, the exhaust gasses are hotter near the engine and a cat needs to be hot to work properly... If we removed a cat from a vehicle and put it into a chamber full of carbon monoxide, nitrogen oxides, fuel vapours (HC's) and air it wouldn't start breaking those chemicals down unless we also heated it. Cats have heat shields because when the chemical reactions are taking place they make even more heat. If we fit a cat near the engine (1st inline) and another cat behind that cat (2nd inline) the 2nd inline cat will be fed gasses from the 1st cat and if that 1st cat is hot the gasses leaving it will be hot... Hot enough to start chemical reactions in the 2nd inline cat? But if we decore the front cat and leave the 2nd cat fitted in the exhaust the same distance from the engine as it originally was then reactions might never start occuring. People who decore front cats but leave rear cats in place might have more luck at emissions tests if they work the engine hard enroute to the emissions test because that will help heat the 2nd cats and get the reactions working but the MOT man might not start the emissions test straight away and even if the engine isn't turned off the rear cats could cool down to the point the reactions slow down and emissions are not clean enough to pass the test.
If you decore rear cats now, then decide to decore front cats, then you won't have any cats fitted and won't pass an MOT emissions test. And of course there are some Elgrands that had rear cats decored, front cats left alone (the owner thinking therefore front cats are intact) but unknown to the owner one or both front cats has failed since the rear cats were decored... So the owner thinks the rear cats are decored and front cats are intact when actually rear cats are decored and one or both front cats effectively decored themselves because they broke up, in which case again it won't pass an emissions test because at least one side of the exhaust has no cats.
There are many different takes on what to do about cats on Elgrands. Some just decore rears, some just decore fronts. Some remove their rear cat sections but don't decore rears, instead they store the rears and buy/fit aftermarket straight-through (no cats) sections of pipe in place of the rears, then they can always remove the rear straight-through sections and refit the original rear cat sections just before MOT, then after MOT swap back to the straight-through sections. Some do the same but also decore front cats. Some fit aftermarket front cat sections. Some decore front or all cats and convert to LPG, if a vehicle is presented for MOT running on LPG the emissions test is far less stringent while at the same time emissions from the engine itself are lower, so a vehicle presented for MOT running on LPG will pass the emissions test with no cats fitted at all (and if there are no cats fitted there are no cats to fail at anytime).
I haven't said much about prices but below I'll attempt to list what can be done, effectiveness in preventing engine problems and effectiveness at passing an emissions test in rough order of cost, all numbers out of 10 with 10 being most effective / most expensive.
Method / cost / engine protection / emissions (MOT) effectiveness
Do nothing / 0 / 0 / 8 . MOT emissions effectiveness is not 10 because there is potential for future cat and engine damage (either of which can increase emissions) but while there are no problems this will be 10.
Decore rear cats leaving front cats in place / 1 / 5 / 7. Vastly reduced the risk of engine damage but there is still a strong risk. MOT effectiveness is slightly down from doing nothing because when we did nothing we still had the rear cats fitted and working besides the front cats. No parts need to be bought but a bit of work to remove, decore (some bashing and welding) and refit.
Straight-through rear sections / 1 / 5 / 7. Much the same as decoring rear cats. Rear cat sections removed and (these pipes) refitted. Parts need to be bought but there is usually no welding or fabrication involved, a simple remove and fit.
Decore front cats leaving rear cats in place / 5 / 9 / 5. The front cat sections have to be removed, decored and refitted but no extra parts need to be bought. It is a pita to remove and refit front cat sections, there's also the bit of bashing and welding to do.
Fit aftermarket front cats / 8 / 8 / 7 . Expensive to buy plus much of the same work involved as ecoring front cats because front cat sections need to be removed and refitted (the same pita job), not usually quite as effective at cleaning emissions as standard front cats that are working properly.
Convert to LPG and decore front cats / 9 / 9 / 10. I've rated the cost of converting to LPG and decoring front cats at 9 because we are rating initial costs. But of course LPG also gives the ability to run on cheap fuel and if you save enough money on fuel the cost eventually becomes negative when you've saved more money than you spent on the install. I could have cheekily rated LPG conversion cost at -400 because indeed running on LPG over the years has saved me a lot more than that lol. But even if you only do 3000 miles per year you could reckon that costs come down from 9 to 8 after the 1st year, 8 to 7 after the 2nd year, etc, until you get to around 4 or 5 years when cost becomes zero, then for any later years cost becomes negative. Maybe even after just a year total costs will be lower than if you'd opted to have expensive new aftermarket front cats fitted. I'm biased because I convert them to LPG but even if I was in a completely difference business and not fitting LPG conversions I think this would still be my choice and it would be a no-brainer choice. The only other option I would consider would be to decore front cats and leave standard rear cats in place. The LPG conversion would be a no brainer for several reasons, it is the only choice that allows it to pass a legitimate emissions test with no cats at all fitted and I like that having no cats fitted means that no cat will ever fail on me. I do keep on top of maintenance with my cars but the idea of preventative maintenance involving removing exhaust components that are working fine at the moment to replace with expensive components containing platinum that will work nearly as good (not better) than the standard components I'd be removing and seeing no other benefit from it other than not having the engine self destruct doesn't bode well with me, I would much rather spend similar money on something that will equally or better protect the engine whilst also add extra functionality like the ability to run on much less expensive fuel and/or double the range it can be driven without needing to refuel and/or add the ability (for say a camper converted van) to run gas appliances such as cookers etc from the same external tank that supplies the engine with LPG fuel without having to carry a gas bottle inside (that might cost more per litre for gas in the little bottle than cost of gas for the big external tank).
Also of course there are combinations of things that can be done. For example some owners will have rear cats decored as a kind of put-me-on first measure before going on to fit expensive aftermarket front cat sections. We could tally the cost of multiple work, so rear cat decore 1 + new front cat sections 8 comes to 9. This would also affect the other numbers (protection and emissions), but no combination of things that can be done will exceed engine protection or ease of passing MOT emissions of converting to LPG and any cat will fail in enough time / enough use, even a new aftermarket one. I'm not saying failure of an aftermarket none ceramic cat will be as bad for the engine as failure of the standard front ceramic cat but if/when they fail it could still be bad for the engine and with a duff cat you'd be back to failing the MOT on emissions again until you'd replaced it at the same expense as the first aftermarket cat you bought. Some owners have decored rear cats then realised it would be safer to decore front cats, but if they're not running on LPG they won't pass MOT emissions testing if they decore front cats because that would leave them with no cats fitted, so they've had to buy intact rear cats to replace those they had decored, then hope it will pass emissions test... I.e. They paid to have front cats decored (and even if a dealer did it 'for free' the dealer could have sold it to them for a lower price if they didn't decore rear cats, it is the customer who ultimately pays), then paid for front cats decore, then because they had no cats at this point and were not converted to LPG they had to pay for intact rear cat sections to replace the rear cats they already paid to have decored... which is why I don't agree that good dealers should automatically decore rear cats without explaining all the options or asking the buyer whether they want rear cats decored. If a garage/dealer charges say £100 to decore rear cats they can make more than they charge because they can weigh in the cat material that has value because it contains platinum, even if it's done for free they can weigh in the cat material... Put it this way, there are not many who have the policy to automatically fit rear straight through bypass pipes AND pass the intact rear cats on to whoever buys the Elgrand. Any new aftermarket cat has to be made, it wouldn't be very cheap to buy even if it didn't contain platinum but it does contain platinum and will be expensive to buy, then it needs fitting and before it can be fitted the original front cat section has to be removed (price of the new front cat with it's platinum plus most of the pita work that is necessary to decore front cats).
