[Voltvo]

I think you may be correct on identifying a faulty sensor. If visually there isn't a buildup of dirt or some other solid that could block the ultrasonic sensor then I would definitely suspect a problem. If it is happening more frequently on cooler mornings then maybe some moisture or condensation is getting into either the sensor itself or the connector. I'm not sure what kind of maintenance you could preform yourself to either fix it or check it, but I wish you luck at the dealer!

You could try to see which sensor it is by getting a friend to block each one one-by-one and see which ones have that reduced functionality. Maybe with it narrowed down to a single sensor the issue may be more obvious.