bubby

Hello! We bought a X-reg ford galaxy with front and rear parking sensors today. When we went to look at it for the first time we asked the owner to show us how the parking sensors worked and they were working perfectly. So a week later we went and collected the vehicle. However: when the engine is switched on, nothing happens expect if we are reversing the light on the parking sensory switch flashes and it makes a short noise. After this the noise disappears but if we are going slowly the light still flashes but it does not make a noise. Even is we switch the parking sensors on when going towards the garage, the light flashes but there is no noise. Not sure what the problem could be as the system seems to be working ok and it seems to be a problem with the sound. Also as it was working when the previous owner showed us, it is difficult to say that it was faulty before. But how could it have developed a fault in a week? Thanks in advance for any advise.