OK, I know this has been a long time in getting to a solution. I am not sure I have one yet, However, I have not replaced the catalytic Converter, nor have I replaced the O2 sensors, However, I am seeing the light come on less frequently. Here is what I have done.

Purchased my 2003 xtype in December and loved it. Then my CEL came on with a P0420 code. I hoped that the cat was not bad and ran some Cataclean through the system, twice. Then I ran a can of BG44K through the system. Still had problems. I ordered two new o2 sensors that are still sitting in their boxes because I could not figure out how to get them in. Too tight of a squeeze between the engine and the firewall.

All this time I have been running premium fuel through the system and seeing minor improvements. Then, the other day I was out in the middle of nowhere and they the only gas station for miles had no premium. I was worried, but decided to use the Higher octane fuel which happened to be ethinol free. After a full tank my CEL light has only come on once or twice. It is shutting off on its own and is more off than on. This is something I have not seen in the last 8 months. I filled up again today with no eth gas and will be watching closely. Only spent 4 cents more per gallon than premium, but if it takes away the check engine light I will be thrilled.

While there my 16 year old boy asked if I really needed to buy the most expensive fuel. I told him yes, beccause it will save me from a much more expensive Catalytic Converter replacement.

Take a look at my thread on the UK site Code P0430 Catalytic Converter

Once a Catalytic converter block has been destroyed, it wont ever recover. My advice would be to either take no notice of the EML or fit a new Converter. I have learnt that the hard way.

Sounds like you have an intermittent issue, but if I was you I would still change the O2 sensors. There is plenty of information here on how to do that. The type of gas might have temporarily helped, but you have had this issue for quite awhile. Fix the underlying issue and be done with it. That is what will help best to keep from having to replace your catalytic converters.

Finally changed both o2 sensors on for the p0420 code and it looks like that was the problem. I guess the downstream sensor was going out. All is well and passed inspection today.

I have a bigger problem same code p0420 but I've replaced both cats and changed all 02 sensors twice still have the code. I took it to 3 shops and they have no clue what to do I think I might have to pay for a dealer diagnostic

Hi Eric,

I feel your pain. In my case I replaced the Down stream sensor and cleared all the codes. My p0420 code never came back. I have heard some negative about after market cats though. Are your cats OEM. BTW, this code is only for the cat and O2 next to the firewall.

From your Intro I thought you have an X350? If so, even though P0420 is also a Cat related issue on that model, posting in a thread in X-Type forum is misleading.

Graham

did you use oem sensors.....also it could be a gasket leak
mine was a gasket leak ,it never showed its face again

watch this video

I have a 2007 vanden plas and yea I bought 2 cats from rockauto. You think the car just dosent wanna accept the aftermarket cats? If not I dont feel too bad I only paid $170 per cat. Any ideas on where I can get OEM cats for a reasonable price?

Hi, I have similar issues with my XJ6 from 2003:
P0430 comes as current fault and P0420 as pending fault (readen out via torque).
But additionally my fuel trim values do not look nice, something about 9% off. I have checked it for air leaks and sprayed on all manifolds and could not see a change in short trim fuel values, renewed the PCV valve.
The only difference I could see, if the engine is cold it adjusted the short fuel trim to +15% for a short while and than it went down again. long trim values are 5 and 9, but they are different at idle, than the car runs lean and the ecm adjusts more fuel.
I could not hear a broken exhaust manifold.

Ive seen on youtube a test about storage capacity of the cat and will try to do that tomorrow, to check the health of the cat and I will try to do a leak check, which will not be very easy (I don't have a fog machine).
For me it could look like a exhaust leak.
Also an interestint point: If I clear the lamp, it doesn't show up for the next 30 minutes or longer, but if I start the engine in the morning, it is just 10 minutes and it is there and does not go off.

Does anybody have another idea for me, where to search?

Hi Elegancec,

You may want to check the XJ6 boards since I believe this is under the x-type or (x-400) board. However, I will tell you that if the p0420 is going to be a problem at or before the cat according to a mechanic I spoke with yesterday. Also, your trims will fluctuate on a cold engine. The ECU should take that into consideration and not even start measuring until they stabilize. As for the p0420. I replaced my O2 sensor and that helped for a while. I am now getting occasional codes. I am going to check to see that there is not a leak at the O2 sensor and also that the O2 sensor is still good. I may have bought a defective one. I hope I do not have to replace the Cat since that is a hard one to get to on my car.

Good luck!

Hi, this is a screenshot short after prewarming, you can see the red curve is much more oscillating than the green one. When the system is really warm, the red and green curves are much more stable, only the red one fluctuates slightly.
After this, I did the storage test of o2: Feeding propane (in this case I have used brake cleaner) to the engine, so it is saturated and just keeping running. If you turning it off, the yellow and purple curve did not immediately turn back, but I could see it on the red and green curve, the red one jumped down two seconds before the green one, which means it can not store o2. So it looks like, my catalyst has really an issue. Maybe cleaning makes sense but if the active area is gone (Aluminium oxide is used to increase the surface), than I need a new one.
https://cimg9.ibsrv.net/gimg/www.jag...683092ceb3.png

https://cimg2.ibsrv.net/gimg/www.jag...247e1053cb.png
Graph withLPG Mode in the midde

https://cimg4.ibsrv.net/gimg/www.jag...3fea2c1df5.png
Perfect from different car 4cyl

https://cimg1.ibsrv.net/gimg/www.jag...696b1a4151.png
Today gasoline

Once again, this is the X type (x400) forum NOT XJ6........