Greetings All,

Has anybody ever had any luck saving a marginal catalytic converter? On my ’02 V6 S-Type, I’m getting the dreaded P0420 code intermittently. The car had been running fine until a misfire episode two weeks ago. I had loaned the car to my son-in-law for the day. Great guy, loves my daughter almost as much as I do, but knows pretty much zero about cars. On a hundred mile round trip (for a VERY important job interview, so I was glad to help), the CEL had started flashing but he is such a nervous driver he barely noticed it and didn’t realize it meant the cat was overheating. He doesn’t remember when it started, but I don’t believe it was flashing the entire trip. As great a guy as he is, I don’t want to make him feel bad so didn’t press him for many details.

I fixed the misfire (oil in the spark plugs wells on the RH cylinder bank) but am still getting the P0420 on a random basis. The sporadic nature tells me the cat is still working somewhat, but not consistently. As an honorary Scotsman, I don’t want to change an expensive part if not needed.

Would a different driving technique be of any use? I’m usually pretty easy on this car, but am wondering if working it hard for a few days might help. Or maybe adding some (insert name of miracle fuel additive here) would be just what I need. The cats are both about 4 years old and I never had this code before. The car just passed emissions testing shortly before this happened, so I have some time to experiment before opening the wallet for the one guaranteed fix.

 

No. Replace the cat. Once they overheat from excess fuel they are beyond any fixing.

The good thing is you can get aftermarket units that are much cheaper than Jaguar.

I would not put a used one on either. They are a long term wear item and you never know the history of the used stuff. Might be fine and might be junk.
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Thanks for explaining that, since buying our 03 4.2 we have had this problem. So
now I know that beyond getting the rear suspension work done I need new cats.
What a money pit this was.

 

As a follow-up, I wanted to pass along my experiences with a bottle of snake oil. Call me Mr. Vegas, but I thought it was worth gambling on this:



I purchased it locally, and the clerk said I could get a refund if it didn't work. Much to my surprise, the stuff worked! Sort of. For a while.

I followed the directions carefully, adding it when the tank was down to about 1/4. I then drove it for about 10 miles, and filled the tank. It threw the same P0420 code a couple of times during the next week, and that was it. Before trying this stuff, the code was being written about 80% of the time. After adding it, it was down to about 10%. I definitely could have got the car through emissions testing if needed.

Unfortunately, the effect didn't last permanently. With the next tank (no new snake oil added), the percentage went up to about 30% or so. Per the reviews on Amazon, many people recommended a second treatment so I tried that. Once again, the occurrences dropped to approximately 10%, but would rise again on subsequent fill-ups. It's faulting about 60% of the time now.

So I'll soon be looking at a new cat, but thought this info might help somebody strapped for cash and needing to pass emissions. In some cases, it may be enough to bring a tired converter back to life, judging by the Amazon reviews.

I'm not really sure of the product's chemistry. It smells like toluene or xylene. Judging by the price, it may be made from unicorn tears, I don't know. No guarantees for anybody, but in certain cases, it could save you a lot of trouble. Other manufacturers make similar products, so consider that, too.

 

Introducing toluene or similar products into the fuel system may cause damage to other components such as the fuel pump and injectors.

In my experience, when the MIL is on and the codes stored are P0420 or P0430, the corresponding catalyst must be replaced. Adding VOC products to the vehicle may produce other problems.

 

Much happiness to report. I'm going to close the case on this one. Before pulling the trigger on a replacement cat, I decided to replace the upstream O2 sensor on a hunch. The P0420 code, referring to catalyst efficiency, is determined by sampling "dirty" exhaust entering the cat, as measured upstream. The sensor sends out a reading and the PCM instantly adjusts the air/fuel ratio to compensate, constantly switching back and forth. The switching rate upstream (fast switching is basically good) is then compared to the switching rate downstream of the cat, where the switching rate should be slower to indicate the cat is cleaning the exhaust as designed. Since sensor switching tends to slow as they age, I tried a new upstream sensor to increase the difference. Please note I kept the old sensor downstream, as slower switching there is good.

Bingo, no codes for over a month now! Initially, right after replacing the sensor, P0420 did come back two times, but I didn't disconnect the battery during the replacement, so I think the PCM had a little trouble comprehending the change caused by the new sensor. After that, no more codes. The suspect cat may still be slightly degraded due to the original misfire/overheat situation, but it's still working well enough to satisfy the OBD-II system.

Also, please note I changed both upstream sensors, not just the one on the affected side. My thoughts are that O2 sensors are normal wear and tear items, and should be replaced periodically. Luckily for me, they definitely helped. As a bonus, a minor cold stumble is now gone, too.

More of my rambling philosophical thoughts on the subject were offered in this related thread, for what it's worth:

https://www.jaguarforums.com/forum/s...x-hmmm-151396/


Please note that thread pertains to a later model car with wide-band upstream sensors. My car is the earlier model with the narrow-band sensors.

 

Good repair. How much were the 2 O2 sensors?
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I installed Bosch #15716 sensors. They were $24.45 each on Amazon.

 

A quick follow-up: The marginal cat is still marginal, but after two years on the new upstream O2 sensors, my wallet and I are still happy. The car still occasionally sets the P0420 code, but very rarely. The car has been through Oregon's emission testing twice since the original incident, and passed with flying colors.

One curious thing I've noticed: Occasionally the car will set the code several legs in a row. It's always after filling up the tank. Once I burn off that fuel, the fault typically goes away several cycles after refilling the tank. There definitely seems to be a correlation to fuel quality. Here's the odd part, though. I almost always run Chevron, purchased from the same station whenever possible. All I can figure is my car's marginal cat is right on the edge, and minor variations in fuel chemistry are just enough to set off the fault code.

Oh anyways, if you are getting a P0420 or P0421 code, I'd be willing to gamble your money on a new pair of upstream O2 sensors and see what happens.

 

Thanks for the follow-up!

 

If you always use the same fuel from the same station how can it be the 'fuel quality' that is the cause of an occasional fault?

 

Karl,

Consider modifying a spark plug anti fouler (HELP products) to accept the downstream O2 sensor & screw assembly back into the exhaust. This will trick the OBD system. This might give you a bit more time out of your marginal cat.

Some may laugh but it's a proven technique and stay away from the "miracle fixes in a bottle". Might actually do more harm than good.

 

Heck if I know. All I know is what I have observed. Somebody recently mentioned that most fuel brands come from the same basic raw stock within a geographic area, and the only difference is the additive package that is added when delivered to the station. Maybe that mix ratio changes slightly depending on the delivery man's mood, who knows? I know my #1 cat is right on the edge so it probably doesn't take much variation in chemistry to set off the code.

 

It's not the fuel.

That was me but you slightly skewed what I said. Look elsewhere for the root cause of your problem.