Originally Posted by mcglups
Background...I love my 2005LGT (5 speed) and it has 150K miles and the back left has a high speed shake that no one can fix. The P420 CEL comes and goes because of the cat sensors and the entire body under the car has minor/moderate rust and sooner than later I'll need to replace the entire exhaust system, never mind that eventually some part will fail. I drive this car very hard each day, it begs to be red-lined a few times a day and I do my best to deliver. But... I can't handle the high speed shake and don't want to be dumping +K cash into a car that is only worth 6K.
So the main problem is the high speed vibration. Your short term solution is to find a mechanic who knows what he's doing. It's not like there is an infinite number of possible causes to sort through here. Bent wheel? Tire out of balance, flat spotted, cupped, etc? Have you tried swapping the LR wheel to another position on the car and seeing if anything changes? Blown shock (look for oil seeping from it, or give that corner of the car a good push downward and see how it reacts)? Disintegrating control arm bushings? Condition of brake rotor? Wheel hubs can also bend if you hit a pot hole or curb hard enough. Condition of axle shafts?
P0420 could be a fouled sensor rather than the cat itself being bad. That's not too expensive to fix, and you can buy spacers to get the sensor farther out of the exhaust stream to reduce future fouling. Don't know if there's anything that can be used to try cleaning the sensor, never tried. Can't that code be eliminated and readiness forced with open source software?
IMO, if you generally like the car and the engine runs well, fix the couple of nagging issues and then keep driving it until something more catastrophic happens.