s11swc

Hi all - new to this site but sounds like a great resource. Would welcome any advice - my front/rear parking sensors have ceased to function - I get a single long bleep when I select reverse, but otherwise they don't bleep at all, regardless of speed/closeness of hazard. Has anyone had a similar problem? I noticed some advice to check for 'ticking' noise from individual sensors, but can't hear anything. Should the engine be running/car in gear to test this? Any thoughts welcome, cheers !