How reliable are they? Mine seem to work intermittently. Would the amount of muck and dirt on the bumper effect their operation? And if there was a fault, would it show up on VAG.com?

Several years back I was involved with a project where the company I worked for designed an aftermarket reversing sensor product. It actually did pretty well and picked up a 'Best Buy' award in one of the big car magazines.

I think this experience has probably put me off reversing sensors for life!!

I don't know what the VAG / Skoda system is like, but we found they were often effected by 'noise', so other electrical equipment could occasionally cause an intermittent blip when reversing.

They also sometimes get ground reflections when reversing over bumpy ground.

We also found that each sensor had slight tolerances and the circuit board would need to be tweaked for each sensor. This was often not done correctly by the manufacturer, causing some intermittent issues.

Dirt on the sensors is also another definite cause for them having problems; the recommendation is to wipe them regularly with a damp cloth to keep them clean.