Despite not having a cat engine code for a couple of years now [cleared once and forgotten], I've been having a variety of problems recently relating to the exhaust system: a lambda sensor needing replacement (bank 1 = easy side) which I successfully replaced, followed almost immediately by a CEL for bank 2 (difficult side).

I had maybe a week or two of incredibly good acceleration, followed by the bank 2 CEL and now terrible acceleration - worse than ever.

Before I go out and buy another lambda sensor though, I think the readings of the pre-cat versus post-cat sensors [shown on the attached graph] are pointing to a failed cat.

Does anyone agree, or am I barking up the worryingly expensive tree instead of just getting down and dirty with bank 2 replacement?

 

You can check to see if a cat is working using a digital thermometer. Compare the temps of both cats. If 1 of them is noticeably cooler than the other the cooler one is not working

 

Unless you have an actual "catalyst effieciency below threshold" code followed by a
"Bank 1 or Bank 2", I would just replace the hard side oxygen sensor. Chances are that the heating element inside the oxygen sensor has either high resistance or it's an open circuit. If you want to know for sure, you need an ohm meter, disconnect the upstream sensor, put both of the probes on both black wires of the sensor connector. If there is no movement of the ohm meter, then it means the heating element is open and needs replaced. Besides, even the "newest" 03' X308 is 13 years old and chances are you have over 100K miles on it and would really benefit to have it replaced anyways. Both of the downstream sensors are basically catalytic convertor monitors and wouldn't benefit you much unless the heaters burned out in those...

 

I'll give the hard side a go in that case and see how things pan out.

Forgot to mention that the graph shows the post cat sensor reacting almost immediately when I blipped the throttle but the pre cat reading not moving. I believe that's a sign that the cats are kaput.

Although there's probably a lot to do with the CEL light & the zeroed out lambda sensor pushing the ECU into some kind of default mode.

 

Since the pre cat sensor isn't moving, that means that sensor isn't functioning properly. Just replace it and it'll be fixed.

 

I am certainly not an expert on this, but my recent experience on my XK8 was the pre CAT wideband sensors (new Bosch units) stayed very close to a 1.0 +/- 0.1 or so, but the post CAT would jump around a lot when I blipped the throttle. The CAT efficiency monitor passed fine and for my CA LEV category that means 90% efficiency. Non LEV I think will pass CAT efficiency test at 80%. I had thought the post O2 readings were a "bad" sign, but we ran a tailpipe test too just for the heck of it when I passed OBDII smog check and it looked clean.


The other thing I find confusing is that on all other vehicles I have worked on the secondary O2 sensor has always been downstream of the CAT, but on my XK8 they are located in the CAT and about 1/3 of the way down the body. I have also seen where Jaguar refers to the secondary O2 sensor as a Catalyst sensor.