Please Note:
(1) This Does Not Fix A Defective O2 Sensor,
(2) No...O2 Sensor Is Included In This Offer.

 

Well... good luck.

The stated causes for codes P0420 (and P0430) according to JTIS are:

HO2 Sensor disconnected
HO2 Sensor to ECM wiring fault
HO2 Sensor heater to ECM wiring fault
HO2 Sensor heater failure
Upstream HO2 Sensor failure
Downstream HO2 Sensor failure
Catalyst failure

And you've got one of those...

The codes/CEL not appearing straight away is pretty normal. On the cars fitted with aftermarket cats it can take several drive cycles and some miles until the code pops up.

 

Thanks Cambo..

HO2 Sensor disconnected
HO2 Sensor to ECM wiring fault
HO2 Sensor heater to ECM wiring fault
HO2 Sensor heater failure
Upstream HO2 Sensor failure... I don't see how 1-4 above are likely.If the upstream O2 was bad wouldn't I get a code and have a light on for that? Would the engine run so good and fuel trims be OK? I'm still learning but was under the impression that the upstream O2, TPI , MAF , Intake air temp and MAP now control the fuel mix.
I do think Jag V8 was correct the first time that very likely the cat is not functioning as designed (toast) but it flows free and I don't mind tricking the PCM to thinking it's OK..

 

The extenders can work for some situations as has been said.

My Ford went over 10k miles giving 0420 off & on 'till the upstream sensors finally got so bad that it failed smog. When I replaced them the fuel economy became better than ever, they were already going bad when I bought it and continued to slowly deteriorate. The O2 sensors don't cost much more than these extenders if you shop around, maybe even less. If they're the problem they will eliminate the code, protect the cats from a rich mixture, and save you money on gas. If you're determined to just fool the system without knowing the cause you'd be better off with the electronic MIL eliminators, you can buy them or make them for a couple dollars-google "MIL eliminator" for details.

0420 can be on & off on any car depending on driving conditions and the cause of the fault. On my Ford a long highway ride wouldn't trigger it, but around town it wouldn't take many miles but a couple days, I do think there are drive cycle delays at play. I could reset the computer and drive almost 400 miles to Az with no light, then after a few in-town miles it would come on, did this several times. Every car I've worked on with 0420 I've reset and the code seems to come back after a few days with no obvious relationship to miles. In every case it's been O2 sensors except one that had an obviously broken cat, no point diagnosing and playing around when it's rattling.

I like the suggestion of swapping sensors left-right, there's a free way to know if it's only showing one bank below efficiency.

 

Other than our little grammar detour (Guys, let's be careful with those apostrophe's), we've been trying to tell you those extender thingies are probably a waste of time and money. Might help, might not. They certainly don't fix anything, just mask symptoms. So yeah, we are guilty of ignoring your original question, at least part of it. I wonder if this is how a doctor must feel if a patient begs for some wonder pill they saw advertised on some slick TV commercial.

The general consensus is new upstream sensors are the most likely fix. Besides turning off the CEL, they may also improve performance and fuel economy. I've had an intermittent P0420 on my '02 for several months. I finally got around to changing the upstream sensors last week and so far no CEL. A cold stumble problem is also gone, too. I haven't racked up enough miles yet to conclusively say it's fixed, but so far it's looking good.

Tell ya what, I'll make you a deal. I'm putting my money where my big mouth is. I bought 2 Bosch upstream sensors on Amazon. Cost $25 each. If you're going to change one, do both at the same time. $50 total. That's $20 more than you're planning to spend on those extenders band-aids.

Try a pair of brand name sensors first, instead of the extenders. If the sensors don't fix it, I'll mail you a crisp $20 bill to cover the difference and then you can try your extenders. PM me with a mailing address if you try the sensors and the code comes back.

 

Thank last two replies.. I was told early on " the whole O2 system is Denso" so I bought that brand for the bank one downstream. Should I stay with Denso and replace the other three sensors or just the one in front of bank one? The shop I took it to that is supposed to be Jag Certified said the downstream O2 would fix the code/light and listed it as things to be done so I went with that but did it myself. Then I heard it was the cat that is still suspicious and if it is the cat all the O2 sensors won't fix it.. Also it runs so good I hate to change anything.. Great mileage and performance. It has new plugs and coils because it had a slight skipping at idle when I bought it that also adds to my bad cat suspicion. I'm just thrilled with this car..

 

Cats will generally outlive O2 sensors at least 2:1 or more. Unless the engine has run out of tune a long time or something weird, the cats should pass 200K easily, generally a lot more. O2 at 100K is questionable, the ones on my Explorer were acting up by 70K and finally got bad enough to force replacement about 10k later. When I got the Explorer it was running on about 5 cylinders of 8; plugs & wires bad and arcing all over the place, air filter so clogged it was sucked into the upper half of the housing, MAF sensor coated in crap, etc. I wasn't able to get it over 60MPH on the way home and very slight hills dropped it to 45-50. After fixing everything and it running normal I got the 0420. I assumed the bad running conditions had damaged the cats, but it passed smog and ran fine so I just dealt with it. I bought a pair of those extenders planning to install them but before doing so I found out my upstreams were shot. Once replaced the code never came back. MPG always seemed about normal until the end when the sensors went really bad, after replacing them my MPG went up about 2 average on the highway. When they crapped out the computer readings looked perfectly normal, sensors were responding just as I'd expect of good ones. Gas mileage was so bad I burned almost 1/4 tank driving back & forth to the smog shop just a few miles away. The sensors should have lasted a lot longer, I believe the miss-firing engine killed them. I later learned that passing too much unburnt gasoline to them will indeed kill them.

 

well then let me QUANITIFY that I have used the extenders from big daddies on many many cars including my highly modified Nissan without cats and NO CODES again so yeah I guess they really Dont work do they

 

Work to fool, but what happens to emissions? How much each of CO, HC & NOx will these cars tend to emit when extenders are used (with bad cats)?

 

And lets be clear here, its not A FIX for a bad part. But to have the 02 read a more efficient exhaust stream after the cat. Lets say the EPA says the cats have to scrub the exhaust enough to maintain 93% efficiency or better(whatever the number is) and it goes to 92. It will turn on the check engine light for that cat. Now does it mean the car runs badly or losses MPG? NO. it meens there is a increase in emmissions above what the EPA has set. you know kinda like VW and now others are being found out too. You know the EPA that was just involved in polluting a the river in Colorado with millions of gallons of heavy metal waste from a mine and turning it yellow.

this works just like Viagra in increasing what you can no longer do or LESS EFFICIENTLY as you age. I doesnt FIX you if you're broken, it only helps with you AGE ISSUE and NOT OPERATNG A PEAK EFFICIENCY IN THAT DEPT

 

Thank you all again I really wanted to know if they have been tried by anyone here and do they work? I stated early on I don't mind fooling the sensor or the Koch Bros owned EPA that I trust about as far as I can throw a piano.

 

In the immortal words of Kent Dorfman, "This is great!"

All sorts of comments zinging back and forth! The Koch brothers, the EPA turning a river into performance art, rogue apostrophe's, a bona fide wager with cold hard cash, piano tossing, Viagra, and so on. All over a simple problem that can very likely be fixed, not just patched, for $50. It just doesn't get any better. Keep up the good work guys!

I am remiss in my previous statement. I should have said: The general consensus (with the exception of the highly esteemed Brutal) is new upstream sensors are the most likely fix.

Zenfly, you've politely asked several times, but with the exception of Brutal, few have tried the extender band-aid. With precious little collective experience, you're not going to get much more in direct response. It's your call which route you take. I'd politely suggest you're down to the point of deciding whether to fish or cut bait. And by the way, my friendly wager still stands.

Just curious, do you prefer the pills or lotion? Not for me, of course, a friend was wondering.

 

Aand I'll add something else when looking at cats. IF you buy cheaper cats than OEM there is much less precious metals in them to function for long properly. This is why a metal recyclers only give $5 each for them vs $50-150 for OEM cats
I Also monitor up and downstream 02 to see if they're cycling correctly and up to speed. SELDOM do I need to replace 02s. but cats. I know I know theyre like a spark plug and yes they should be changed with time, but like going to a Dr. The tests say you have a bad kidney so he doesn't replace your heart does he...

 

Any update? I was wondering if I owe you $20 yet...

It's been a week since I replaced both upstream sensors on my '02. Previously, it had been throwing a P0420 code about 75% of the time. The RH cat had been heat-damaged several months ago when my son-in-law borrowed the car for a big job interview and #1 spark plug decided to misfire on him. Poor guy, knows practically nothing about cars, and had no idea what the flashing CEL meant. He put over 100 miles on it like that. I fixed the misfire but kept getting the P0420 code.

After finally deciding to do something about it, I changed the two upstream sensors last weekend. (Remember, I'm still running the old downstream sensors.) The code seems to be gone now, but in the interest of full disclosure, it did show for two instances earlier in the week shortly after installation. The upstream sensors were the only thing I changed (other than a coolant leak I fixed at the same time) so I'm very happy with the results. I only saw the code those two times, and haven't seen them since. I've put a lot of miles on the car this week, making 9 round trips of approximately 50 mile each.

Hope this helps. Keep us posted.

 

"9 round trips of approximately 50 mile each." is about how far mine goes with the light off so please keep me posted on that.
Both upstream sensors on mine are putting out the correct voltages.As far as I know from all my recent studies that is all they can and should do and my iCarsoft has never shown any code other than P0420. I don't even know what upstream codes are yet. I'm getting 28+ mpg highway at 70-80mph and 18 around town with some spirited driving that seems in order. Were the voltages within specs on your upstream but you changed them anyway ? Did you have upstream fault codes ? My old school brain has a hard time replacing parts that seem to be working and while the car runs so good I'm not in any hurry to change anything. The highest rated Jag shop in my area stated that the sensor I replaced was the culprit after his test.(oops) The Midas muffler shop wants to replace both cats (no surprise) for only $600 with Walker brand but so far so far I have just turned the light off and thoroughly enjoyed this amazing car.
____

 

Assuming you only get the 0420 on one bank, why don't you just swap the upstreams L to R?

On my Ford they were showing good voltage and I had no O2 codes. 0420 was a permanent fixture for a few years. It's been at least a year since I did the upstreams and did nothing else, no codes since. I had another car with 0420 on one bank so I replaced the downstream with no change. The sensors had well over 100K miles so I decided to replace them all, 0420 gone. I've monitored O2 data from cars with 0420 before and after replacing the O2s and couldn't see the difference, but the computer obviously could. How bad an O2 can be before an O2 code is given will depend on the computer's programing. A sensor that's off but still giving acceptable readings will go undetected. The level of acceptance will depend on the computer's programing as to what it'll take for it to realize and alert you that there's a problem. Even then the code it sets off may be for something else because the computer may not know the cause, just the effect.

 

No upstream codes. P0420 was the only code I had after fixing the misfire on that side (Oil had collected in the spark plug wells). I watched the up and downstream sensor patterns but really couldn't discern anything that screamed "change me".

It was only after I read up on exactly what triggers the P0420 code that I got to thinking. Paraphrasing the factory manual, the sensor switching rates are compared before and after the cat. A good upstream sensor should be rapidly switching. The downstream sensor switching rate should be slower, indicating the cat is doing its job. If the downstream switching rate approaches the upstream rate, the cat is becoming less efficient, hence the code. I put on my thinking hat and realized O2 sensors become lazy as they age. My theory was to keep the old, lazy downstream sensor and put a new one upstream. This would increase the difference in the switching rate. Fingers crossed, but so far it seems to be working. It's just a theory, but many others have reported equal success.

When troubleshooting, one important thing to keep in mind is that computers are incredibly smart, but also incredibly dumb. They can monitor and control systems that would overload our simple brains, but they can only do what they are programmed to do. For example, an OBDII computer can easily determine if one or more cylinders is putting out less power than others and bring this to our attention. However, try something stupid like filling a gasoline tank with diesel, and the computer would probably have no idea why the engine stopped running. There is no "diesel instead of gasoline" monitoring system, so the poor computer would be out of its element there. Any human mechanic would hopefully catch the diesel smell right away, but not so with a computer. As a professional mechanic, I absolutely love computerized control and fault monitoring systems, but you still have to know how to interpret what they are telling you, being aware of their limitations. And sometimes, like in my case, it's just educated guesswork, combined with laziness and cheapness on my part. (Years ago, I was taught a good mechanic is a lazy mechanic, and I have taken that advice to heart. Seriously...)

When dealing with most mechanical issues, I always highly suggest making sure the basics are in order. On another forum (Ford pickups), I caught the tail end of one discussion as a non-mechanic type guy threw up his hands in disgust and was preparing to give away a free old truck he was trying to resurrect for use on his farm. He had himself convinced all sort of horrible faults were present and it wasn't worth his time or money. All it needed was new plugs, a cap and rotor, and fresh fuel, but it took lots of convincing to get him to try. Now with our newer vehicles, O2 sensors fall under that same category of normal wear and tear items. An O2 sensor has to be REALLY bad before it gets flagged. As far as my car's computer thinks, I changed a pair of perfectly good upstream sensors. It never flagged them. But on the other hand, a long-time cold stumble is gone. Mileage has improved, too, from about 24 to 26 mpg. I don't have the official maintenance schedule handy, but a general rule of thumb is to change O2 sensors every 100,000 miles. You don't have to wait until they throw a fault code. If you don't know when they were last changed, it can't hurt to try them.

I realize a week is still a short time with recurring codes. All I know is my car was setting a P0420 most of the time before I changed the upstreams, and now it only happened twice since then, and that was on the second day. I think that may have been because I didn't disconnect the battery, so the poor computer had to make some rapid adjustments with the new sensors. I'll report back if the fault returns.


Welcome to OBDII land. If you're really concerned, mark the old sensors at removal and reinstall them if anything gets worse, although that isn't likely.



This might work and won't cost anything but your time, but I'd still recommend new upstream sensors. You want to get the maximum difference in switching rate pre and post cat. That's why I recommend new sensors upstream and keeping the old ones downstream.

 

The upstreams won't be switching as they're widebands.

 

Are the downstream sensors narrow band? I know they have different part numbers from OEM Denso..I just read that wideband are much more sophisticated than a narrowband sensor and designed to be used as a tuning tool. I also read that the downstream were there only to keep tabs on the cat function and had nothing to do with tuning fuel mix but then I also read that wasn't true and the downstream do effect fuel mix on Jags.

 

Read the workshop manual as that tells you more. Yes those are the older, cheaper tech. They should not switch much if the cats work. But they are used for fuelling not just that - please stop reading anything other than the proper jag doc such as JTIS as you get the wrong info elsewhere.