C5 X7 rear parking sensor fault

[aerodynamica]

Hi all, Old Nim, my C5 has been behaving well lately but the rear parking sensor has gone wrong. When I engage reverse the tone sounds as though I'm about to run out of space regardless of what's behind, even nothing at all! The picture in the display also shows the set of coloured bars as though I'm about to hit something. It only appears to be the middle sensors that behave this way - the set to each L & R side function normally.

When I do begin to reverse towards something however, the sensor begins to work more or less normally so it is only when it's clear behind that it behaves wrong.

Any idea what the nature of the fault could be?

[myglaren]

No experience of it but have read that jetwashing is a common cause of sensor failure.
And very often the remedy is

Spoiler: show

WD40

as that is what it was originally intended to do, not all the other things that it doesn't excel at.

[GiveMeABreak]

Normally if there is a fault with any one sensor the system stops for safety reasons. Diagbox will be able to tell you if it's a sensor fault. Water in the sensors used to be a C5 MK I / MK II issue and as Steve says, was often caused either by pressure washing where the water is forced into the senor seal and starts corroding - or water enters from underneath the bumper onto the loom and rolls back down into the sensor plug wiring.

I've not heard of any MK III sensors failing as yet, but that's not to say they don't.

[aerodynamica]

Cheers, it's not acting like it's failed only that it thinks there's something there. I actually had a thought: there's a tow hook and I wonder if that is upsetting it... might remove it and see if it changes

[GiveMeABreak]

That will do it every time. You are supposed to deactivate the reversing sensors when towing of course, but if you have one of those non-retractable tow balls, that's your issue and the system is doing its job as intended.

[aerodynamica]

my god, do you think? Here's how it presently is.

If it's the cause I'll feel daft

[GiveMeABreak]

It will be detected as an obstacle - the sensors are ultrasonic, and anything within 30 cm of the sensors make it impossible to detect, so if the obstacle is already there then this will cause the alarm as the towbar is within minimum detection range.

[Paul-R]

Yup, that'll be the problem.

Is the bar and ball detachable? It looks like there's a twist lock on the side of the bar.

[aerodynamica]

Yup, that'll be the problem.

Is the bar and ball detachable? It looks like there's a twist lock on the side of the bar.

I think so, I'll be honest I've not played the tow bar much attention.. I don't tow anything and it was on the car already. I'll have a look later as I'll be doing some work on the car today for the first time in a long time...

[aerodynamica]

Ok! The tow hitch doesn't actually remove, it stows away up under the bumper, as does the plug for the electrics. Quite a neat mechanism although completely rusted. It took 2 hours to fold it away but it's away now! And it seems that this has cured the parking sensor anomaly!

No more tow hitch

[aerodynamica]

.. my attempt at removing the rear corner spheres however, failed. Like every other sphere on this car they are so tight that you'd feel the subframe will snap instead of the breaker bar. In fact the 1/2" drive extension the breaker snapped..

[Paul-R]

Good news about the towbar.

Not so good news about the spheres though!

[Hell Razor5543]

I would consider using some lith-moly grease on the towbar, both where it pivots and on the ball (to making towing easier). Then pivot the towbar in and out of position a few times, to help the grease get into the join. Then it shouldn't take so long to stow it away in the future.

[Paul-R]

I get the feeling that having been folded up out of the way that towbar isn't going to see the light of day again!

[aerodynamica]

I get the feeling that having been folded up out of the way that towbar isn't going to see the light of day again!

Correct!