Uff-da, it looks like I'm out of my element here. I did most of my research with a set of PDF manuals for '02, which had the narrow band sensors. Sorry for the confusion. I'm sure you mentioned the model year, but I overlooked the difference.

Zenfly, you may be at a dead end here. You've got to make your own decision about how to proceed. Sounds like one shop said your cats are bad. Another shop suggested the downstream sensors, which you already tried. I could hedge my bet and say the light would stay out if you replaced the engine, complete exhaust system, and all O2 sensors, but that might be a bit extreme. I highly suspect you could narrow it down a bit from there...

I don't know what criteria is used to throw the P0420 code with a wideband upstream sensor. I'll still go out on a limb (and wait to be corrected ) by suggesting the computer still uses the same basic logic and watches if the downstream sensor sees cleaner exhaust than what has entered the cats. Since the wideband sensors don't switch back and forth like narrowbands do, then maybe the computer just looks at an absolute value upstream for that part of the calculation.

This is all (somewhat) educated guesswork on my part, so please remember how much I'm charging for my advice. The upstream sensor could still be at fault, as it is of unknown age and condition. As previously suggested, you could try switching the upstreams side to side and see if the code follows. Unfortunately, your wideband upstream sensors are a LOT more expensive than the narrowbands on my '02. I replaced both upstreams and have been very happy with the result.

Just remember, with some issues like this, the troubleshooting isn't always clear cut as one would hope, as evidenced by what two shops have told you. It's your call what to do next. I don't think anybody has a clearly definitive answer for you.

 

Quote from GT42R post #96 in My Green STR. But so far I've been turning the CE light off about once a month. I'm still considering a $13 part instaed of a $$$ cat.

"No CEL's with the o2's setup this way...

In my past experience of trying to eliminate p0420-cat-efficiency codes due to high-flow-cats/cat-deletes, the 'sparkplug defouler' trick at the secondary o2 would work well. A reduction of exhaust flow directly over the sensor was enough to fool that particular emissions-only monitor...

Recently, I've found that merely having it plugged in and reading free-air, is enough to avoid throwing any p0420 style cat efficency codes, or any other code, so for now, that's how I've left it, and it's been fine for the past 5 months. I would consider threading them back in along with a defouler/spacer, to have it looking proper undearneath, come inspection time.


It's not uncommon in aftermarket tunes/programmers, to have the option to disable the secondary o2's altogether. Will not effect engine operation in any way, and will test perfectly fine on the latest obd2-based emissions testing procedures... I'm fairly certain eurocharged can disable my secondary o2's if I so specify the next tune around..."

 

It'll just mean fuelling is a bit wrong which will inevitably reduce the life of the cats and cost more $$$ for fuel. May emit more pollutants (mainly HC I expect). Up to you (and the laws) if those are OK.

Might well fail our MoT (we do exhaust i.e. tailpipe emissions tests using a gas analysis machine).

We have a crowded island with many (tens of thousands of early) deaths due to pollution so our laws try to stop this sort of thing (VW please note).

 

Thanks always... Let me explain.. I live in the USA home of the Koch Bros. who dump millions of tons into the air every year and pay millions to our politicians. I don't feel guilty. I live in Florida "The Sunshine State" where you won't find any solar collectors or wind power because the energy companies here own the politicians.. Make sense of our govt when the solar incentives are in NY where they have 4-5mos of snow on the roof and then rain half of the spring. Are you surprised we have no emissions testing here in FL?

 

Er... now you've posted that, it makes "sense" (I'm wanting another word there!)

 

I said I don't feel guilty.

 

I still have not changed anything except to turn the P0420 light off every 300-400mi. I'm getting 29mpg and 18 city with great performance. Are people actually replacing upstream sensors with no upstream fault codes? It seems that all the light is telling me is I may not pass inspection in NY or CA.

 

Yes.

And what I've always read is that the computer is looking at the voltage difference between them rather than switching rates, but my sources weren't engineers so I don't know.

 

I suspect it's the cat not the sensor so would check the cat first and only suspect the sensor if the cat was good.

You can use OBD to tell if the upstream sensor is working, but as you have no code it probably is. Before replacing it I would definitely check it!

 

I have an iCarsoft tester that only shows the P0420. I've haven't had any indication that the upstream were not functioning.. The car is amazing and I've never been happier with anything in my life.. Harleys, Mercedes..boats..you name it..

 

I don't know of any test you can do to determine if the cat is working at its expected level. A dead or clogged cat is fairly easy to check with temperature readings.

I went through this and all 4 sensors seemed to be doing their jobs even when the upstreams were finally so bad that it failed smog miserably and was getting horrible mileage. The smog tech checked them as well and was surprised; the engine was running rich and the O2s were acting perfectly normally but were lying. The computer didn't know to throw a code for them and was reacting to what they were telling it, add more fuel. He called a Ford specialist friend about it and the guy immediately said "change the upstreams". We questioned him and told him they were working, he insisted they were bad. I couldn't believe it would be the sensors so I put in a used pair I had laying around and it was cured. It's been at least a year now with no issues. When I changed the sensors I passed smog and my mileage came back, even a bit better than before they crapped out hard. I had no expectation that they would also cure the 0420 but they did.

My O2s started giving 0420 at about 70k miles and finally crapped out harder at around 85k. The sensors I put in, and am still running, had well over 100k. My car and the one the sensors came from had totally different lives; the PO of my car was very abusive and the other car was well maintained.

My experience was with a Ford and each brand seems to have their own computer quirks where they'll do different stuff in response to the same faults. If Ford was involved in the Jag computer it would make it more likely that they'll act similarly. The O2s causing 0420 I've experienced on many makes. I've cured a lot of 0420s with new O2s, none of them have needed cats except one that was rattling.

Another Ford quirk that people can't believe 'till they see: OBD1 Ford truck with a rough idle and/or late harsh shifts with no other obvious issue. Fuel filter; been there done that at least 6 times, air filter can do similar. Some of those trucks had had the trans rebuilt to no avail. One of them got a 'major tune-up', the trans rebuilt then entirely replaced, then a new computer with no change. I changed the fuel filter that Firestone couldn't remove, they tried like hell judging by the damage and then left it but charged for it anyway. The customer wouldn't believe me because he'd had it replaced, so he thought. I finally told him I'd forgo the service fee (I'm a mobile mechanic) if it wasn't the filter. He was so happy he gave me a $100 tip when I cured it and stopped using anyone else.

 

The O2 sensors drift as they age, reading leaner which makes the ECU run the engine richer. I was told once that this is a deliberate thing as it's better for them to drift lean than drift rich. Running your engine rich is safer than running it lean.

Which is why people often report improved fuel economy after changing the upstream sensors.

 

It's just what happens more than deliberate engineering. The voltage is created by a chemical reaction, not terribly different than a thermocouple on a heater or a battery. As the metals degrade their ability to create voltage does too. Lower voltage tells the computer that the engine is running lean and it fixes that by adding more fuel. As long as that sensor is putting out an acceptable amount of voltage the computer will listen to it and not detect a fault. Since we're only dealing with millivolts in a low current system it's easy to have a weak sensor that the computer thinks is good. Much like using the spark plug spacer thing. It doesn't change the exhaust that hits the sensor, but the sensor's reaction is changed enough to fool the computer into thinking everything is good.

 

I don't entirely buy that as the S-Types use the downstreams to fine-tune the trim so would back off the adjustment supposedly made for the aged upstreams.

Still, changing them is an option.

 

It makes no sense to use the downstreams for tuning and I don't buy it. If anyone has proof of it I'd love to see it. Regardless, if a sensor is lying within acceptable parameters the computer will be altering the fuel mix incorrectly. Whether that information comes from the up or down stream. If it's using the downstreams for tuning that means that any change in the catalyst performance will force a tuning change; again, makes no sense.

 

3 proofs:
1. it's detailed in the workshop manual
2. there are tech articles aimed at auto industry professionals which describe it
3. someone on the UK S-Type forum changed his downstream(s) and did indeed see improved mpg

I can't follow your supposed logic but that's neither here nor there.

#1 does it for me as Jaguar had to put effort in to state it. #2 works for me also.

Actually #3 had me considering it might be so.

 

Well that's proof, thanks.

The only reason I can see to alter the mixture based on the downstreams would be to maximize catalytic efficiency.

I don't know what of what I've said has you confused about my logic. I've repeatedly fixed 0420 with nothing but O2s. I've lived through bad O2s throwing the mixture to extremes even when they appear to working fine. Basing the fuel mixture on post cat. exhaust must be for helping emission, for best engine tuning it needs to adjust based on what's coming from the engine unaltered, from what I know.

 

Yes it's true that the downstream sensors are also used to manage fuelling, they are not only for monitoring catalyst performance.

On the Jaguar engines this has been the case since the AJ-V8 was first introduced in 1996. And it applies to all vehicles since.

Trying to find a proper explanation is not so easy, but I did get something from a Denso document (the Jaguar gasoline engines used Denso ECU's up until around 2013 when they switched to Bosch, it also applies to the Bosch cars).

So this is it:



Basically the downstream sensors are used to control the "skip" which is like an over or under correction when there is a rapid change in the AFR.

Which is why the slow &/or incorrectly reading downstream sensors create problems with Air Fuel Ratios during throttle transitions. Some more info in this thread Xj 4.2 remap

You can see the dyno charts with the AFR plots in that thread, also the pictures of the "minicats" or extensions which we took off the car.

I assure you that the downstream sensors make a considerable contribution to the fuelling on these cars.

 

So is it using the secondary O2 signal as kind of a buffered signal to fill in between spikes? Just trying to get a good grasp on the idea. It's also interesting that the computer alters ignition duration based on O2 readings, never heard of that. Do you know if this is a Jag thing or a common OBD2 thing?

 

I don't know about that in any detail so I really can't comment.