I will, actually, have to replace my catalytic converters in the near future. I had a cylinder go out on my original engine and the shop believes that, during that time, the cat's got clogged up with un-burned fuel and burned/un-burned oil that was exiting through the burnt valve in the cylinder. I am starting to experience a lag in power under acceleration in certain gears, but I am also getting a check engine light at times. Most of the codes have to do with the sensors before and after the cat's so I had a shop check it out and they confirmed that the cat's aren't flowing like they should.
I have been doing some research on replacement cat's and if you get them from Isuzu they are not cheap ($1,400 per side). My local shop wanted to charge me about $1,400 to cut the old cats out and weld in new (universal) ones, but after looking around a bit it looks like there are a few companies that make the entire left and right cat assemblies for the VX (Benchmark, Bosal, Eastern Catalytic) and the prices aren't bad ($300-$400 per side). http://www.autopartswarehouse.com/sk.../BO099014.html That is one example, but I have found them for less money. These are the complete cat assemblies so there is no cutting and welding involved. You just unbolt your old ones (which might require some heat on the nuts and bolts to get them loose) and bolt up the new ones (with new gaskets).
Hope that helps.
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