Originally Posted By: Wolf359
Originally Posted By: andyd
Yah,change both on a V6. Cat condition hasn't direct effect on the engine except that a sick engine will kill a cat. My firsy '88 BMW passed emissions at 335K miles on its original cat.
You mean both upstream ones right? As mentioned earlier, it's the upstream ones that are in the stream of things and wear out. The engine uses them to adjust the fuel mixture. The downstream ones are after the cat so if everything is working right, they don't get much abuse and can last much longer. They're not switching much and they're there mostly to tell you when the cats are bad.
And if things go south on the cat, like a P0420 or P0430 efficiency code, you can block the sensor off from the exhaust entirely, its there for an electrical connection only.