Had a VAG trained guy run a diagnostic on my octy Rs yesterday to try to get to the bottom of these random spells of hunting and stalling it's been suffering from. After the last bout it registered misfires on three of the cylinders and a weird reading on the front lamba sensor (probably related and not the root cause) but I also got the same 'implausible reading' fault on the throttle body angle sensor that I had with this previous TB - so far that appeared to go away after it was swapped. It also pulled a 17705 on me, but one thing at a time.

I was wondering - what exactly does it mean by implausible? Is it saying the angle sensor isn't feeding back numbers in the right range, or that the position doesn't seem to be correct with reference to something else.. and if so, what? Given that clearing the fault and re-aligning the TB brings the car immediately back to running perfectly I can't help thinking that it's not the TB itself at fault, but that some other aspect of that control loop isn't doing its job. Could it be that the component causing the problem is being switched off by the ECU for safety after a fault is detected, suggesting that it's one of the things that has the ability to regulate the idle?

Just for reference, the TB itself is spotless inside and the valve actuates perfectly. There are no vacuum leaks in the obvious places and all of the measuring blocks show the sensors working either at, or above expectations.

I'd think that "implausible butterfly angle" means that you're showing as flowing too much or too little air and fuel for the throttle angle being reported. It could still be the angle sensor, but that would return an "TB angle out of limits" message if I'd been programming the system, because sooner or later it would try to give you a value that equates to "less than 0 or greater than 90 degrees".

But that's a s/w engineering view, not a VAG diagnostics one.

Implausible Signal normally means the ECU is not seeing what it is expecting to see.

I'm guessing this would apply to you -

00670 - Ross-Tech Wiki

That could well do, cheers. I was suspecting a wiring fault because of the sudden yet completely reversible nature of the problem. Suspicions were raised further because on two of the occasions it's failed the first turn of the key resulted in a click, battery light and no attempt to turnover at all. I'm wondering if something is grounding out, or there's a problem that's getting boosted by moisture - thinking back, I reckon every failure has been on or after a damp night. If the valve position is being misread as wider than it actually is, then that could plausibly trigger a 17705 fault from the perspective of the turbo and DV couldn't it as the airflow would be lower than expected?

Implausible is a weird word in context, isn't it? It's relative, and so you would imagine that it can't highlight a fault without reference to at least one other factor. I could imagine if, for example, the MAF was detecting a lower airflow than the valve position could technically allow then that would mean one or the other was implausible. Having a code description with a big gap where the other factor should be is far from helpful - I agree with you Ken that a lot of the codes would probably benefit from a clearer description.

If the TB is out of alignment then effectively the ECU doesn't know where the TB flap is and thus can't accurately control it so it may very well cause rough running.

I have no doubt that if you were to get yourself a cheap lead and a $99 licence you will save the money in the longer term.

Have the vacuum pipes been checked under the inlet manifold?

I'm becoming more inclined to get the money together for that - now I've seen it working I'm quite intrigued by the initially spannerless approach to sussing out what's wrong I'm very aware that my own curiosity vastly outweighs my ability though, so I'd have to be very careful about poking around

Lummox - yup, that was one of the first things to be checked. All of the hoses that tend to split have been replaced quite recently. When you realign the TB the car idles perfectly at 700rpm, the needle barely flickers, (unless you peer really close - the diagnostics showed a deflection of about 15-20rpm) and it'll stay like that until the problem reoccurs. It's sureal to have it running absolutely perfectly without the tiniest hint of a problem and then for it to suddenly lose the plot overnight, returning to perfect health after a realignment. Even weirder that it's the second TB to pull that trick.

I'm wondering actually, based on VRStu's earlier suggestion - where does the cabling for the TB go? Is it left along the inner wing and round the back to the ECU? If so, could it have been knocked or otherwise damaged by the garage who did the cambelt before I took delivery? I wasn't too impressed with their approach from the start, so it wouldn't surprise me. My other half said that she thought it happened when we'd had wet and windy weather overnight, so I wonder if something is getting shorted out?

Ross do you remember the problem with StuartJ's car and you mentioned a relay, do you think that might be a cause? I can't remember which one it was although I could point to it

Found a very interesting article concerning the Audi S4 DBW throttle system, which I assume is pretty much the same as the one in the Octy RS.

Fourtitude Forums: How your drive by wire ME7 MKIV 1.8T/VR6 Motronic works.

It refers to implausible signals, but in the context of a difference between the readings of dual redundant potentiometers connected to the throttle pedal and also inside the throttle body. If I'm reading it correctly, it implies that an implausible value fault occurs when the values given by the two pots don't tally - ie. when one has failed. When this happens it puts the car into an emergency programme and uses the value from the other pot to control the throttle until it can be sorted.

The particular programme depends on the severity of the fault, from one angle sensor down, to a throttle drive failure and finally a combination of the two where the postition and status are unknown.

In my case, it seems to be that the second angle position sensor in the throttle body is kicking up a fuss, but that's happened on two different TBs now and seems unlikely. Let's assume for a moment that the sensors are both OK. The signal showing the position of both sensors is part of a closed control loop leading back to the ECU, so would I be right in thinking that the only way an inplausible signal could occur is if the ECU was at fault, or if the wiring sending back the position of one of the sensors was failing and either earthing out, or showing high resistance? The implausible signal could stem from the sudden low value on one of the wires, leading to a complete inability of the ECU to rationalise the duff signal with the correct one. I would assume that the dual redundancy built in would mean that the signals were carried individually rather than multiplexed, so it would come down to a single wire or pair. If this was the case, and the car does actually sufffer more in the wet, then that might be the clincher.

Sorry to go on, but this is like thinking aloud for me

Final questions then:

- what precisely does a TB adaption do? Looking at the throtttle body it doesn't seem to hold any particularly complex electronics, so am I right in thinking the ECU just runs it through the range of movements, records the limit positions and then issues control signals accordingly?

- Does anyone had a wiring diagram for the cables connecting the TB to the ECU?

After a couple of months of being fine, this has happened again - cutting out the traction control and ABS on an icy and extremely cambered backroad was interesting. I'm going to get it scanned this afternoon but suspect the 'implausible angle position sensor 2' code will be back.

Just out of interest - does anyone know how common issues are with the wiring and connectors between the TB to the ECU? I'm now pretty much certain that this second TB is fine, as I now believe the first one was, and as the car runs perfectly after an adaptation I doubt the ECU is in error either. That's doesn't leave much between the two

Not even ten days after the last reset and this has reoccurred.. this time, just when reversing out onto the road. Always implausible throttle position sensor 2, nothing else.

Low level question: Does anyone know if position sensor two is assumed to be in error if there's a discrepancy between the two, or does the ECU monitor the trend for each and report an implausible signal if the input from one deviates relative to the other?