Oxygen sensor 1.4 AUA location

eminegee

Member
Hi ,
Don’t have the access to a ramp
just trying to find the post cat 02 sensor location on my car
found the front pre cat one under the heat shield at the front of the engine
but can’t seem to find the 2nd one
Could anyone help
Many thanks
 

Edwrai

Member
Its quite easy to find its not far from the end of flexi pipe veey roughly below where the gear stick is inside the car
 

mihtac

New Member
Hi guys,

I bought a replacement sensor for this, and had it installed in a service (it was a Bosch). Trouble is it came with the plug unmounted, so they had to wire them. And I suspect they wired it wrongly, as the sensor isn't functioning properly (status "not ok" in VCDS, and the car struggles at idling - is runningrich).

Do you know where i can find a wiring diagram for this sensor? Engine is 1.4 AUA.

Thanks!
 

eminegee

Member
Hi
I tried a universal sensor on my 1.4 AUA in the front manifold were we had to adapt the wiring
This wasn’t successful
Ended up buying an NGK brand which another club member recommended to me
This was just the same as the original Audi one which was taken off (same plug on the end and wiring was the same length a direct replacement)
There about £100 approx
But it’s been on the car for nearly a year now👍No probs
Regards Mark
 

mihtac

New Member
Thanks @eminegee - if I were to change the front o2 sensor, I'd get an NGK too :) . For the other one, I hoped Bosch would be a reputable enough brand for an aftermarket sensor.

I have this situation now, for which I'd like some advising, pretty please.

Car symptoms:
- on cold start, everything is fine - starts fine, idles fine, accelerates fine. STFT (short term fuel trim) and LTFT (long term fuel trim) are 0
- once the engine/02 sensors warm up, LTFT jumps to +35 and STFT oscilates around -10 / -15. overall/added up, it runs rich
- also once warmed up, idle revs oscillates a lot (on the 500-1500 rpm range) - to the point of engine stalling. Works fine on acceleration

Tried the following:
- disconnected the o2 POST-cat sensor (bosch one) - the LTFT no longer jumps to +35 when the engine warms up, but still oscillates a lot on acceleration
- also disconnected the o2 PRE-cat sensor (so both were disconnected at this point). Car no longer oscillates on idle, but hesitates on acceleration/WOT. overall, it's drivable, though. Also no STFT or LTFT corrections (both values stay at 0)


Not sure what to do next. I'd buy a new pre-cat sensor (NGK), but it's a bit pricey, and I'd like to make sure the current oone is faulty. I'm thinking it no longer corrects the fuel trims as it has no o2 sensors to read on - so it ignores that aspect. Dilemma is, though: when it was correcting the air/fuel mix, and failing at it (as the idle was so jumpy) - was it because the sensor was giving inconsistent readings? or was it because whatever it was using to correct the mix was not working properly? (is it the throttle body? did take it out and clean it, seemd fine really)
 
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ajsellors

A2OC Donor
Hi,

the post cat sensor is mostly there to verify that the cat is working correctly. The ecu compares the readings between the pre and post cat sensors to see that the catalytic converter has done its job. You will get a fault and an engine light if it is not working correctly but that won't affect the fuel trim.

The fueling is all controlled by the pre-cat sensor. This is used to measure the actual result of fuel combustion and adjust the fueling by applying a real-time offset to the value returned by the ecu map. The ecu tries to maintain the combustion in a narrow range either side of the ideal but lambda sensors degrade over time. The effect of this is that this lengthens the time delay between the gas hitting the sensor and the corresponding reading coming out as an electrical signal. This is sometimes reported as a o2 sensor "aging" fault. The in built heater can also fail and this is needed to keep the temperature up in the operating range when the engine is idling. If the sensor is on the way out then it can be lagging and so the ecu's fuel adjustments are being applied too late and so end up not maintaining the correct combustion. By applying corrections too late the actual value can already have deviated too far out to be corrected and so you can end up with wild oscillation between extreams.

The ecu however can only apply a maximum deviation from the map fueling. If there is something fundamentally wrong with the engine then this may not be enough to ensure correct running. E.g. air leak, blocked injectors, poor compression.

The short term fuel trim is a reading of the real time adjustment made by the ecu based on the reading from the pre-cat sensor. Assuming that the pre-cat sensor is working correctly, there is an allowed deviation either side of the map that corresponds to a correclty running engine. I can't remember off hand what the values are (they are shown in VCDS) but +35 way out. The ecu maintains the average short term fuel trim and stores this in the long term fuel trim. The purpose of this is to use when the engine is cold after starting as the lambda sensors don't start working untill they warm up so this value is used to run the engine until they do. If the short term value is way out then so will the long term average. There is a separate fault for that "Long Term Fuel Trim Additive Air". That just means that on average, the engine is running out of specification. You can clear that fault and the engine light will go out but if the underlying problem is still there, the average will be out of the allowed range and the fault and light will come back on.

If you disconnect the pre-cat sensor then the ecu won't know what's going on. The fuel trim values will be 0 as then ecu has no reading and the engine will be run only on the programmed fueling map. This won't correctly compensate for load etc. and so although the engine will run, it won't run well. Note also that with the sensor disconnected on average the engine will run too rich and long term this will kill the catalytic converter as it will overheat due to burning off the excess fuel.

Your pre-cat lambda sensor probably isn't working correctly if it is old, but that may be a contributing factor rather than the only cause.
A cheaper check would be to first verify that all air and breather hoses are OK. Particularly the oil separator at the back of the engine. I have just needed to replace mine as it had cracked in two! Some of the hoses behind the engine can also rub through and just generally crack from old age.
The throttle body is probably OK, its worth a clean if bad but probably isn't causing your problem.
Make sure that the MAP readings are sensible, if they are then its probably OK.
If the EGR was causing problems then the ECU would probably flag that up specifically. The ECU checks the operation of the EGR by monitoring the pressure changes via the MAP as the EGR opens and closes.

My bet would be a combination of an "aging" pre-cat lambda sensor (part of the problem) and clogged fuel injectors (most of the problem). The fuel injectors on the AUA are easy to remove and refit and can be sent off for cost effective professional cleaning. If its got this bad, fuel additives won't be enough to restore injector performance, they need ultrasonic cleaning and spray pattern testing. You can also find relativly reasonably priced "OEM" injectors on eBay.

regards

Andrew
 
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mihtac

New Member
That's a very extensive reply Andrew, appreciate it!

I have no engine lights in dash (I'll check if it lights up on ignition key, hoping the bulb ain't deactivated lol).

I have reset the DTC codes several times - with the post-cat sensor connected, though, the LTFT just jumps straight to 35 regardless. And it does it as soon as the engine/sensor is warmed up, and without the STFT being at such high values previously. I'm suspecting it's doing this as maybe the post-cat sensor is showing values so much off that it straight goes for "maximum fuel trim"? 35 seems to be the maximum.

I've already ordered the NGK pre-cat o2 sensor (from autoscout germany, though - so expecting the delivery to take up to a week or so). Wondering if I should take out the injectors meanwhile, and send them to an ultrasound cleaning or not. Will think about it.

The hosings seem fine, for what I could see (checked the ones connecting to the throttle body - one with the sensor in-line as well, is it an air temp sensor?). The oil separator you're referring to is the one at the back of the engine, driver's side (UK) and towards the low end, right? connected to a.... pressure regulator was it?

Other than that, how do I gain access to other hoses at the back of the engine, and are there others? Seems tight around the area :)
 

ajsellors

A2OC Donor
That's a very extensive reply Andrew, appreciate it!

I have no engine lights in dash (I'll check if it lights up on ignition key, hoping the bulb ain't deactivated lol).

I have reset the DTC codes several times - with the post-cat sensor connected, though, the LTFT just jumps straight to 35 regardless. And it does it as soon as the engine/sensor is warmed up, and without the STFT being at such high values previously. I'm suspecting it's doing this as maybe the post-cat sensor is showing values so much off that it straight goes for "maximum fuel trim"? 35 seems to be the maximum.

I've already ordered the NGK pre-cat o2 sensor (from autoscout germany, though - so expecting the delivery to take up to a week or so). Wondering if I should take out the injectors meanwhile, and send them to an ultrasound cleaning or not. Will think about it.

The hosings seem fine, for what I could see (checked the ones connecting to the throttle body - one with the sensor in-line as well, is it an air temp sensor?). The oil separator you're referring to is the one at the back of the engine, driver's side (UK) and towards the low end, right? connected to a.... pressure regulator was it?

Other than that, how do I gain access to other hoses at the back of the engine, and are there others? Seems tight around the area :)
If the trim is going to 35, that I think is as far as it can go so that just means that what ever is going on, the ecu can't compensate for it. Even when my engine was as its worst, it wasn't that bad!

The electrical thing in the pipe you mention is actually a small heater element which prevents the breather pipe from icing up and getting blocked in cold climates. Interestingly this is connected to the same fused power feed as the ECU pedal sensors (brake switch and clutch switch) so if this goes bad and shorts as happened to me then you get bizarre behavior on the cruse control and a ecu brake switch fault!

On my AUA which is May 2001 I have the older style "simple" oil separator. This is a more or less rectangular box and the single hose goes to the back of the air box before the throttle. Later engines have a cylindrical unit which has a electrically operated valve I believe. Mine cracked on one of the seams this year and was leaking lots of oil as a result.

I would see what happens after you have changed your lambda sensor. If everything is fine, great, if not, I would check the injectors next if the hoses are OK. When fault finding its best to do one thing at a time and observe the results before moving to the next.

regards

Andrew
 
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mihtac

New Member
Lambda sensor came in, changed it this evening - no change I'm afraid :(

Probably gonna check the injectors next... and the oil separator (and replace the o-rings).

Any other suggestion? Reminder that with both lambdas off, it does not oscilate at all (and the fault only occurs at idle, runs fine on higher revs).
 
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mihtac

New Member
Not really d9n't think so...

Another detail to remember and that confuses me - it does the oscilating with or without the post-cat sensor plugged in. Only thing that plugging in this sensor changes is causing the LTFT to jump to +35, as otherwise it stays to 0. It should run fine with it plugged off as far as I'm aware?
 

mihtac

New Member
Injectors sent to ultrasound cleaning, picking them up tomorrow. There's another thread mentioning that if the rough idle does not happen with the a/c on, it could be due to the fuel delivery - as the engine tries to compensate for the extra load with more fuel/air.

Also ordered injectors o-rings, oil separator o-rings (that's gonna be a pain to get to), new throttle body seal (metal one), new fuel filter..... planning on replacing all during the weekend.

Is there any particular vacuum hose I should check? There's load of hoses everywhere... but they all seem fine.
 

mihtac

New Member
New update... got the injectors back (cleaned and checked profesionally), thought I'd replace the MAP sensor just to rule that out... (and while doing so I discovered it was only held in one screw.... added the missing one). No change, though.

Does seem to happen a lot less if I turn on defrost (and so the a/c compressor) and it seems to happen quite intermitently...

Not sure what the "trigger" is, if tge engine warming up (close to 90 degrees) or just other random factor.

Replaced/checked so far:
- both lambda sensors
- injectors
- throttle body
- MAP sensor

Not checked/changed:
- fuel filter
- oil evap

Bad weather to get under the car for the above... If you guys have any other idea on what it could be, appreciated. Another detail is I seem to have trouble getting it rolling if not careful (engine might die if not revved properly and careful with the clutch release).... but otherwise the car starts extremely fast.
 
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