Diagnose and interpret 156 JTS lambda values - Alfa Romeo Forum
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(Post Link) post #1 of 15 Old 30-10-13 Thread Starter
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Diagnose and interpret 156 JTS lambda values

Hi there,

does anyone have any knowledge on diagnosing the 4 lambdas on the 156 JTS? It's the standard motor system control failure with codes P0420 and P0430 more or less permanently present.

The full history is listed here: Alfa Romeo forum: JTS MCSF, lambdas and coils

Short story: All lambdas changed in 2009/2010, some 40000 km ago. All coils are new, plugs replaced this summer.

So, how does one interpret the lambda values from the broad band precat and "standard" postcat lambda?

Attached photo shows MES plot of bank 1, note: short time after start up (not closed loop).
Precat lambda in red varies from 2-3 volt, with a spike when I give it some throttle (4 volt).
The postcat stays a 0 volt at idle, then 0.7 volt at 2000 rpm. max 0.8 volt.

Never mind that all the plots drop to zero, seems to be a logging error on my old laptop.

Anyone? What do you look for when using MES? Or is oscilloscope the only way to be sure about whether the cats or lambdas are bad?
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File Type: jpg lambda 1 4.okt.jpg (136.8 KB, 82 views)
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P0420 is catalyst below efficiency? Usually means one or both of the cats in the manifold are not working properly. This often happens if the car is continued to be driven with a persistent misfire (failed coil usually), which drowns the cat (haha) in unburnt fuel.

If not, maybe the injectors need cleaning?
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(Post Link) post #3 of 15 Old 31-10-13 Thread Starter
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Yes, sorry, "Cat Efficiency Below Threshold". Back in 2009, the workshop first decided to replace 2 lambdas. The error came back, then they changed the other two. Obviously they did not perform a proper catalyst fault diagnosis. The error has been coming back on and off for three years according to the papers I have, but lately is mostly on. I've had the car for 1 year only.

There is only one record of misfire, and that was last year before I bought the car. 1 coil was replaced. I noticed however that one coil had also been replaced in 2006. So now one coil is from 2012 and 3 are from 2013. There is no service history suggesting any misfire issues in the period 2009-2010. But still, if the cats are shot, this should be possible to pick up by properly diagnosing the lambda readings? If not with OBD, at least using an oscilloscope.

Cleaning the injectors is also something I have read plenty about. But how does one differentiate injector symptoms from catalyst symptoms? Before considering a maniverter replacement I need to be sure that the cats are in fact faulty..
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I have a problem with my JTS having a lumpy tick over, sometimes slight, sometimes quite severe. Possibly some of the information I have may be of use to you.

Information from my Indie:
Measured from another JTS on tick over: pre-cat 2.2V (lean-burn voltage), post cat 0.025V.
Pre cat at around 1.5V is non-lean-burn voltage.

With engine fully warm (fan cutting in & out):

The two states of my engine:
when in non-lean-burn
non-lean-burn.jpg

when in lean-burn
lean-burn.jpg

It is of course a bit difficult to know whether the lambdas are telling you that something is wrong, or that the lambdas themselves are faulty.
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I have read this page which is quite good on lambda/cat diagnosis:
BAT Auto Technical-Professional mechanics giving advice-An Educational Site w/ OBD2 Trouble Codes and Technical info & Tool Store.

"The sensor should achieve a max voltage of at least 800mv and not more than 1100mv. A minimum voltage of 200mv and not less than 0mv should be achieved as well."

Minimum/maximum voltages and reaction times are the ones to look at - but then you really need a 'scope. However, things are a bit more complex when you have a broadband lambda..
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That page would seem to be referring to a vehicle with 2 narrow band sensors.

These links have some wideband sensors info.
Lambda Sensors diagnostic advice & FAQs fault finding and symptoms
Lambdapower - Lambda Sensor information and supplies - Technical Notes Index Page
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Thanks!

But still, I assume the postcat sensors provide the basis for catalyst diagnosis?
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always thought that more than 1 lambda is a waiste of technology and money...
is it possible to run the car properly with just the pre-cat lambda plugged in?
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Most likely the post cat lambdas are just checking that the cats are working correctly, not providing any info to fuel the engine, so they will in theory have no effect on the way the car runs. It's just Euro III regs I guess, designed to throw up a fault code to annoy people into replacing failed catalysts.

Sillyspeed - if the lambdas have been replaced and they are telling you the cat is not working correctly, then I'd suggest your fault is a failed cat. Probably cheaper to replace the manifold with a known good second hand one than replace a couple of lambdas anyway, so it's not necessarily a bad outcome? If the car had a failed coil and was driven on 3 cylinders even for one fairly short trip to the garage for repair then in my experience that can be enough to cause the problem you are seeing.
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Thanks for your input, sprint_veloce. I share the same understanding, postcat lambdas throw errors either because they fail them selves or if the catalyst is not efficient enough. With no noticable driveability issues in my car, this makes sense.

Replacing the cat means buying a new one, probably from belton massey UK (BM91342H | ALFA ROMEO 156 2.0 (JTS) 03/02-01/06 Catalytic Converter | Cheap Catalytic Converters) which only costs a fraction of the original maniverter. It is not a huge expence, it's more the fiddle with replacing it (rusted bolts etc) and the risk that the problem might still persist (e.g. injectors or carbon build-up) that worries me a bit.

Any yes, it may be reasonable to assume that the car was driven to the workshop with a misfire on at least two occasions, as the previous owners were not DIY-ers.
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Yep you certainly wouldn't want to replace the manifold unnecessarily. Do you have a good local specialist who could give you another opinion? The one you linked to is a good price though.
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Quote:
Originally Posted by sprint_veloce View Post
Most likely the post cat lambdas are just checking that the cats are working correctly, not providing any info to fuel the engine, so they will in theory have no effect on the way the car runs.
A common misconception. An unfortunately non source documented statement is here Second O2 sensor read down to 'Rear fuel trim Experiment'. In particular note 'Every vehicle manufacture today uses the rear O2S for fuel correction; even if it is undocumented.'

SillyspeedJTS: That is an excellent price for the catalysts, have added the link to my favorites. According to my old copy of eper the price is £1,800 and prices have gone up over 10% since then. I presume that will be a Chinese copy, but with a 2 year warranty who cares?
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Only a Bosch service center that I have talked to has the equipment to perform a proper cat diagnosis with a.o. oscilloscope. Probably a couple of hours (x £100) work. The alfa workshops only repeat "yeah, you need to replace the lambdas". When I stress that this didn't help for £1600+ worth for previous owners, they say "once we actually had to replace the maniverter for the problem to go away". Not terribly professional...

edit: It may be a chinese copy, but I know that the biggest indy exhaust supplier in Norway uses the same part (around £600): http://svendseneksos-engros.no/index...product_id=817

Last edited by SillyspeedJTS; 01-11-13 at 11:42.
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Sensingsys - I did say "Most likely..." as you are correct it is not clear whether the post cat lambdas effect fuelling. Most info seems to suggest that early Euro 3 engines did not use the post cats but more recent set ups do. JTS being released in 2002 are early technology so most likely I think aren't using the post cat signal, but I can't back that up with anything.

It would be good to know for sure on the JTS though.
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(Post Link) post #15 of 15 Old 02-11-13 Thread Starter
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If you check here: Alfa Romeo Ersatzteile - Italia Ricambi - Katalysatoren-Ersatzteile / Verschleiteile fr Alfa Romeo 156 (932)

you find that there appears to be both Euro 3 and Euro 4 maniverters for the JTS. I have sent an email to ricambi to ask. In ePer both versions come up for my VIN. The Euro 3 (55180214) is the aftermarket one from Belton Massey.
The Euro 4 has the additional code in ePer "268: Voluntary Service (Limits Emission)".

I know that the lambdas changed during 2005, but only the pre-cats. So maybe the new Euro 4 maniverter (if correct) is from 2005 onwards, mainly reducing NOx?, since the Euro 4 emission req's came in 2005? The JTS was available in the GT till 2010, so I guess they had to adjust to Euro 4...

This is a bit on the side reg. the question, I don't have any idea as to whether there was any major updates to the JTS in this aspect. The JTS uses Bosch Motronic MED7.11 for all model years for both 156 and GT to my knowledge. I guess this suggests that any Euro 4 versions did not alter the post-cat lambda injection control. I bet squadra in Holland would know, they have much experience on the JTS.
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