FSI : lambda probe fault code but dealer suspects a defect cat !...?

fbi

fbi

Member
Hi,
My EML light is on again :(
The engine doesn’t run very bad, but it’s not 100% OK either…
It’s when flooring the gas pedal, or when driving uphill on the motorway that it appears clearly that there aren’t anymore 110 bhp under the bonnet. When I accelerate very slowly, then I got the impression that the engine responds better (the revs increase “more freely”).
My dealer scanned the engine for free, and he told me the only fault that was read is related to the lambda probe (I mean the “main” one, in the manifold = G39).
He told me, in his opinion, that he has serious doubts the lambda probe is at fault. He thinks the catalytic converter may be “blocked”…
Well, I am very doubtful !
I once had a blocked cat on a previous MGF, and it was very different symptoms. The engine wouldn’t rev above 4000rpm. It was very muffled.
Here that’s not the case at all. OK, the rev needle doesn’t hit the redline as it should, but the exhaust doesn’t seem to be restricted. I suspect more an engine management issue.
The average fuel consumption is above normal by 20-25%. When I display instant fuel consumption on the FIS, it seems that the lean modes are still activating in low revs/load, at least the instant fuel consumption displayed is low. But when flooring the gas pedal, things get worse (well, that’s normal …to a certain extent !).
According to the documentation, a defective G39 lambda probe makes the injection dependent on a map, rather than an “adaptive strategy” to maintain lambda =1 (when homogeneous mode), based on the oxygen ratio calculated from the lambda probe readings.
I wonder, in the 2 lean modes (lambda = 3 and lambda = 1,55), how does the engine management unit react to a defective lambda probe ?? I suspect it has no effect, since a lambda probe is not accurate in these unusual ranges (far from lambda=1), and that the fuel/air ratio is calculated based on other signals received (induction air mass meter, air pressure meter, fuel pressure meter, …). That’s why I think the lean modes are unaffected, and that my lambda probe is effectively defective.
I see where it is at the front of the engine. Is it possible to remove it easily from the top ? Or does this imply dismantling other things ?
Is there an easy way to test it ?
Since a new one is 180€ + VAT, I don’t want to change it if not needed…
A new cat from the dealer is deadly expensive, but I think I can find an aftermarket compatible one for the third of the price, if I ever had to replace it.

Thanks for reading :)
I look forward to read your experiences and thoughts…

Cheers,

Fabrice
 
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If the cat is blocked, you should also struggle to rev in neutral. The lambda is not cheap, but it affects the engine map, I changed my own and it was £120 from Audi. Did he clear the fault code and it came back?
 
That's what he told, the fault came back immediately.

How was the change job ?
Did you have enough access to work from the top ? No need to put the car on ramps ?
I don't know where's the lambda probe on the 1.4
Are there 2 of them, probably ? Was it the first one that you replaced ? (the one more ahead)
 
pre-cat lambda was the one, the more expensive one that lives in the exhaust manifold inside the engine bay. I only had to undo the cover on the manifold, normal spanner and carefully unplug the probe before unbolting it. Not sure about the 1.6 but I imagine it can't be that different. It isn't the cheapest part but neither is it most expensive. These things do wear out over the years, they live in the nasty exhaust manifold.

On the other hand, I think my CAT is slightly broken and time for a change, no fault code reported but 'sounds' like it.
 
In the 1.6 FSI, the pre-cat is the cheapest one, since the "post-2nd-cat" 2nd probe not only monitors oxygen but NOx aswell.
 
I had the same problem with my 1.6 fsi ended up buying a Bosch one 75 pounds if remember with a discount code got it from euro car parts for the pre cat and fitted myself using a friends ramp from his garage, without will not happen as for me best access was from underneath needed to remove the plastic guard and stuff so impossible without a garage ramp I would say, the post cat one I sourced from a breakers and fitted as well. The eml went out strait away when I started the car after changing both there is a thread on this as I Googled it and found a guide for the post cat exhaust side not the pre cat engine side if that makes sense?
 
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Thanks !
Do you have a reference for the Bosch probe you bought ?
Why did you change both probes at the same time ?
So, will try from underneath, once I'll have bought a new probe...
 
Ok thats not a problem I dont have it to hand as I am at work but will post the part number once I get home. Had to change both due to error of taking off the rear one when I didnt have to :( . I tried to use a set of wheel ramps from the front end only and had limited space underneath, Ended up cutting off the post cat wires to get it off with a ring spanner as I didnt have enough room for anything else. Only to find I took off the wrong one or I ordered the wrong one either way I had to now change both.
 
Hi fbi,

The part number number on my receipt bought from euro car parts is 710443325 costing 71.95
also says customer part 0258007068?

Hope this is of use to you.
 
Part ordered...
I'm looking forward to fit it and taking the car for a test drive !
 
I finally changed my lambda probe.
Not a diffcult job, but it has to be done from under.
Once the undertay removed, it's an easy access, the probe is is just 10-15cm far from the bottom, and requires a 22 key.
To unplug the loom, one has to first unscrew the bracket it is slid into.
The car now runs better, and the fuel economy seems to have improved sensibly.
I still have to test it on a motorway uphill (because there, it was obvious that some bhp were missing…)
However, the yellow light is still on. I don’t know if it will go off by itself, or if the fault has to be cured with VAGCOM.
 
it should have gone out by itself but try clearing with vagcom if that doesn't work check with vag for the fault code and see what comes up before and after the clear of faults listed then re check if light comes back on
 
Thanks :)
That will be my next task !
After 100km, the fuel economy diplayed is still very good compared to what it was before... Fingers crossed :)
 
My "uphill problem" seems to be solved for a while...
But I can't remember if the lambda probe change was the "solution" to that.
Anyway, although the power doesn't drop suddenly anymore when uphill, the delivered power is far from 110 bhp, IMO. It used to pull much better a few years ago...
Maybe another problem is masking the uphill power drop, who knows !

The engine management light is still on, and there are 2 error codes :
one about the "setpoint not reached" for the intake manifold flap ;
and the other about the cat efficiency below limit.

I don't have a lot of time now to give a try at solving all this...
Maybe within 4-5 months.
The car is more than driveable, so no hurry.
Except the aircon/heater blower that died this monday :mad:
I'll dismantle it today and go order a replacement.
Who said "Audi quality" ? ?
 
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