Air Suspension spurious lowering 'implausible' fault code

Hi

A few weeks ago was having trouble with air suspension whereby when left for several days it would lower onto the stops but would pump up again once engine started. I suspected a leak and booked it into local indy. However before I could get it in the air suspension failed altogether and would not pump up.

Indy diagnosed it as failed front offside strut. This was replaced (non-OEM) and all was fine for a couple of weeks. However now the suspension has started lowering of it's own accord intermittently. It does not move when driving at speed or when parked up with engine off however when stopping to park, or reverse into drive or parking space or sometimes when first setting off etc. it lowers at the front to the point where both warning lights flash but then raises itself again to comfort and remains.

I took it back to the indy who was unable to find any problems anywhere with the system and only fault code present was 'implausible signal'. It seems too much of a coincidence for the fault not to be related to the new strut but currently indy is saying this isn't the cause.

My guess would be it's something to do with a position sensor on the new strut though I don't even know if they have one! Certainly seems to be something that is not directly monitored by the cars system.

Any help appreciated.

2005 3.0L Avant Quattro, 148000 mile.

Hey Rods16,

It does sound like one of the position sensors is giving a dodgy signal and the computer is reacting to this. The position sensors are separate to the struts, and there is one on each of the 4 corners. They're obvious when you take the wheel off. A little black box, with a small plastic arm, then a metal rod linking it to the lower control arm.

I don't have any knowledge of them failing or how they fail, and I would imagine that if it was an intermittent connection then it would throw a code for an open circuit to that sensor, not just "implausible signal" (unless it says "implausible signal sensor X"??). But it's certainly possible for the inner workings of the sensor (probably some form of potentiometer) to break down and then give spurious readings to the level control computer.

If it was me, I would connect VCDS and then log the level signals coming from the 4 corners. They are all visible in mm under measure blocks. Then when the issue happens check if there was a sudden spike or change in the reading coming from one of the sensors. I'm hypothesising that one of the sensors gives a faulty low reading, then the computer airs down to compensate, or something like that.

Regards
Mark

Thanks Mark, good to know where the sensors are located. I've now identified that it dumps the air from offside front when steering is put into hard lock, I'm suspecting the sensor is being contacted by the wheel causing a false reading or something. Possibly not positioned correctly when the new strut was fitted, or a fault with the strut itself when steering putting stresses on it.

Ya, that is an interesting issue. Knowing what I know about the arrangement there, I'm going to guess that the air line is being pulled or something, like it's not routed correctly. Or alternatively the electrical connector is being stressed on the sensor.
But thinking about it, I believe it throws "implausible signal" when it simply doesn't understand how it got to where it is. Like if you manual take all the air out of the strut without telling it to do it, like if the air line was being stressed or pulled.
Have a good look in there with the wheel off. The problem will hopefully be obvious. The struts don't turn with the wheels turning, so I doubt it's the strut itself.

An implausible signal is normal a broken wire/switch in a feedback wire.

Do you have the scan to post up the full code?