Anyone know if parking sensors have a known lifetime?

We've got front and rear sensors on our A4 and one of the fronts went bad after just 3.5 years. I bought two replacements and will be getting the faulty one replaced soon but I have a feeling that another may have gone bad. I guess the fronts have a harder life since the get the same abuse as the rest of the front of the car but I would have expected the designers to take that into account. - although front and rears are the same part no so probably not!