Mazda 6 park assist problem.
 

Hi all. I have a Mazda 6, and when I press the Park Assist button I hear 5 beeps. The reversing camera works fine, but even when I drive right next to a wall there is no noise to warn me I am close to something.
When I take it out of reverse, the screen changes and I see a yellow exclamation in the middle of the screen (so on top of the roof of the car on the display) and a yellow exclamation to the left rear. Am I right in thinking this indicates that the left rear sensor is faulty? The car makes no noise at all when I am next to something, so can only assume it isn’t working. I only picked up the car today, so just want to ask your opinions.
Thanks all.