Well when I first got the misfires, a coil pack was replaced. Jag runs very well and has great performance. But I noticed if I gunned my engine before it had time to completely warm up, I would get an error.

An application I use called Torque, displayed the error as a misfire. I cleared the error to see if it would re-appear. The error wouldn't return unless I simulated that same situation.

I noticed after I changed the air filter, the P0430 error occurred. and I cleared to see if it would re-appear, which it hasn't.

My car shows 24mpg average. I am not sure if that is considered bad for a Jaguar with this many miles.

 

My 04 STR has been setting P0430 and P0420 codes off and on over the past year. (Couldn't pass inspection last year.)
I have Magna-flow Cats on the car.

Last year I used some RXP fuel additive, reset the codes, ran the car for about 100 miles, the codes went away, and I passed the test)

A few weeks later they were back again.

I read everything I could find on the codes and finally after reading in the forum somewhere, I decided to install spark plug spacers to move the sensors farther away from the exhaust flow out of the Cats.
(Dorman P/N 42002, drilled out to 1/2" ID)

So far, the codes have not come back yet.

I read that as the cats age and/or you put in aftermarket cats, the O2 sensors have too tight a monitoring range and throw the code when you are just a little out of spec.

The spacers only cost around $5.00 a pair, plus drilling time. You could put them in and see if this works. If not, very small amount of time and effort lost. If they don't work, then you know your Cats are going bad or you have some other upstream issue as others have written.

Joe