engine light

Started By: Terri Donsker on 2013-09-30 07:09:30

I have a 1997 XJ6--It failed inspection because the "check engine" light is always on. Computer diagnostic points to the catalytic converter. I replaced it last year. The engine light came on again this summer, pointing to the same problem. I feel it is an electronic glitch that has already cost me $2000. It is now costing me my car. How can i resolve this?

T. Donsker


The only way is for car owners to start complaining en-mass to get some common sense into these ridiculous California rules. Particulary car clubs and associations. We have no problem dropping bombs all over the world yet we have to have a car smogged which can fail for minute emissions!

Are you positive that it is one of the cat converters? have the oxygen sensors been checked, I know silly question


We need lots more information in order to help you. "Points to the catalytic converter" doesn't help us. What was the fault recorded exactly, both then and now?? Is it exactly the same as before?? Where are you located, is it in fact California?? Who replaced the catalytic converter, and what type was used?? You have to be very careful if you are in California. Most replacement converters ARE NOT sufficient for CA. You have to be sure you get one that is, and that will cost quite a bit more that the generic.

We need to know the fault. Get the code read properly and let us know.


Thank you for your answers. Nothing is wrong with the oxygen sensors. The fault code is p0420. This is the same code I had before I replaced the cat converter. ( Apart from this, there are safety issues. There is play in the tie rods which could lead to front suspension failure. To replace the steering rack with an aftermarket part would run $1500. --and rust repair to undercarriage).

But my question: If I have already dealt with the fault code, must I replace the cat converter a second time because of the "check engine" light, which could--in my opinion, is--be a false reading. How many times should a cat converter fail in 74,000 miles?
I live in NH.

Just my opinion, but I would sure as hell NOT take it back to the same place that is doing your emissions testing!!! Biggest scam in the world to show a customer some "fault", then charge big money to fix it. Use your Jaguar club to find people with a similar car to find out if they know of a "friendly" emissions testing facility.
What a load of BS!!!

I would never replace a converter based only on the existance of trouble codes P0420 and P0430. There are other possible causes for the codes. A reasonably qualified technician can usually get to the root cause of the problem be analyzing oxygen sensor readings, fuel trims, and other data stream results.


Other X300 owners, myself included, have battled a P0420/P0430 mystery for years. In my case the codes only set when the LTTF (Long Term Fuel Trim) hits "-100". Since fuel trim readings are intended to range only from 0-25 and this is obviously an anomaly of some sort. In my case the codes might pop up every few months...or every few days. Totally random.

As I live in an area without emissions testing I've never been forced to research the problem to the bitter conclusion, nor spend any money, to speak of, in doing so.

In any case, don't let anyone replace the converter based only on code P0420 popping up.


Doug is right on all counts. Lots of AJ16 cars throw a cat efficiency fault once or twice a year, especially if they are up in miles. Any leaks [exhaust leaks] are generally manifest by showing a cat. efficiency fault. AJ16 are known for manifold cracks, those usually set P0420 or P0430 faults just like a failed catalyst will.
The problem is these are titanium dioxide O2 sensors, they work well but are very fragile and cannot be diagnosed very easily. Their output can be watched, but it is meaningless to us because there is a calculation the ECM has to do to assess the catalyst. It's called the area difference, and has to do with the separation of the waveforms between the upstream and downstream sensors. We can't tell good or bad just by watching the waveform like we can the zirconium O2 sensors used on other cars.

Doug, we can't really use the fuel trims on our 1995 cars because they are there, but they are not fully implemented on them. They were not technically mandated until the 1996 model year. The 1996-97 cars will show them in the correct form, plus or minus figures. The 1995's show them 0% to 100% with neutral being 50%. Even at that, I have never found them very useful, they just haven't proved very logical. The AMFR and FMFR PIDs have been what I've had to use to diagnose fueling troubles.

Sorry.......long winded! :-)

Steven, thanks for the insight on the fuel trim implementation. I've learned something new for the day ! (I was getting worried....it's getting late!)

Without going into sad details and/or hijacking this thread I'll say that this early OBDII on my '95 has tormented me to no end. Separating cause and effect can be difficult enough even when you trust the readings. When you're not confident that what you're seeing represents what's really happeneing, well, it make interpretation that much worse.


I have a 97 Jag xj6L with engine light on.. confused about what mechanics are saying..some are saying there are 4 cats and 4 o2 sensors but the regular service place quoted me for one aftermarket cat... anyone know the answer here? I saw a post on "just answer" from a jag tech who said there are two cats as part of an assembly in the front that would be responsible for the engine light FO420 code because the back converter is not monitored and wouldn't need replacing... any info on this would be appreciated... thanks! Dee Massachusetts


Yes, the forward or front catalytic converter is actually two converters welded together as an assembly. Much further downstream, under the body of the car, is the 'rear' catalytic converter.

You have four oxygen sensors. Two ahead of the front converter and two aft of the front converter. As mentioned the rear convertor is not monitored.


This was Jaguar's first foray into catalyst monitoring, it was not as fully reliable as they are today. P0420 and P0430 catalyst faults can turn up sometimes with a catalyst that has not failed. The biggest culprit is air leaks in the exhaust system ahead of or near the O2 sensors. The downpipe that has the catalysts has welded joints that can fracture causing leaks, and the AJ16 has been known for the exhaust manifolds to actually crack, also causing air leaks.

I own an X300 with high miles and about once a year a catalyst fault turns the Check Engine light on. I have yet to find a true fault or leak in my exhaust, so I just clear it and press on. I would advise checking for such exhaust leaks, and if none are found, have the faults cleared. If the P0420 continues to come back rather quickly, you can assume you have a weak catalyst. If not, maybe you'll be in the same situation I am, and only have to deal with it after several months of happy Jaguar seat time.

Good luck!

Just want to say thanks to Doug and Steven for the valuable information... I am at the mercy of the service people telling me that the cat is in need of replacing.. emission testing is due in August so I need to get this engine light PO420 issue solved.. I actually told both service places that ran diagnostics that I have "friends" on the Jag Forum and JCNA and what I learned. I would never know if there is a leak somewhere tripping the code or the CAT.. but I am grateful for this information.. love my Jag!

As a follow up wanted to post that I had a Bosal cat assembly installed with new platinum plugs to the tune of 1300.00. Got my new emission sticker. Now I wait to see if this cat will be ok and not set off the "check engine" light.. no leaks found anywhere, o2 sensors were fine, etc. The service people suggested I do a "motorvac" to clean the injectors. Not the spray mist system, the real deal. Anyone have any opinion on this? Thank you as usual..

You'll probably get a few different opinions.

I'm a believer in professional injector cleaning. I've always felt an improvement afterwards.

IMHO the very best method is to remove the injectors and send them to a fuel injector specialist for cleaning and testing. Output and spray patterns can be checked and compared. Marginal injectors can be weeded out.

Next best is the Motorvac method. Similar methods/process might be known by a different name depending on the equipment used by a particular shop. This is an on-the-car cleaning process. It works perfectly well, IMHO.

Least effective, in my experience, are the pour-into-the-gas-tank injectors cleaners.


Doug, thanks again for this info... I will discuss removing the injectors and fully consider all.