Originally Posted by wa3ra

I see those numbers, and they scream bad lower sensor.


The surest test is a tailpipe sniffer, find out what the exhaust actually is.


I was sure I had bad cats, had them sniffed yesterday, and not only are the cats OK, they are better than some new cars, and had been replace with new factory cats sometime in the car's past.


Realistically, a cat should last the life of a reasonably maintained car, especially the factory cats on these things, they are put together very well with a solid honeycomb and a lot of precious metal.


Replacements can be had for reasonable dollars (about $300 for the pair), but they don't have nearly the guts that the originals do.


The front sensors actually look at the fuel/air going in and coming out of the combustion chambers, and if they sense too many hydrocarbons (or not enough) coming out, they will tell the computer to richen or lean the mixture to get the exhaust into expected parameters (fuel trim).


These numbers say that the mixture is as expected.


The back sensors are after the cats, and they sense the ratio of several gases in the exhaust, and if they are 'too high', they decide that the cat is not efficient and throw a code.


Now, our Jags use a high-temperature sensor (like 600º C!) that has a heater to bring the sensor up to temp quick.


The Jag's computer is based on that rapid change.


The heaters actually burn out, meaning that the sensor is still OK, but the changes aren't fast enough. The car still runs as it should, but the CEL comes on because the car decides that the cat is inefficient. In reality, there is nothing wrong with the cat. If the actual front sensor fails, by the way, the trim goes crazy, and the computer will start dumping all kinds of extra fuel into the system, but at that point, you will have a LOT of codes to deal with (and a blinking CEL). Too rich a mixture will damage a cat quickly, and if too much fuel gets into the cat, it can light off in the convertor. That is an expensive backfire, because it will fracture the honeycomb, an the cat will truly be trash at that point.


Denso, the manufacturer of out sensors, considers them a consumable part, and they actually have an 'official' life expectancy of only 30,000 miles in the Jag application. This considers 50/50 highway/city driving.


Again, in reality, the MTBF (mean time before failure), sort of wrong, since they are more 'mileage' based than 'hour' based is around 100,000 miles.


However, here we are 6 to 12 years out, throwing codes like dice in a craps game.


That is why it is rarely the cats, and more often the sensors.


Jag's own DTC tree indicates this; there is an entire list of things to check before the cats.


The 'cheater' gets the sensor tip out of the exhaust stream, so it thinks that all is OK. The downside is that if something does go south, you will lose that warning early on.