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The forward-looking sensor that measures how far away the vehicle ahead is and how fast the gap is closing — normally a radar unit behind the grille, badge or lower bumper, sometimes a laser unit, and often fused with a windscreen-mounted camera. Some vehicles use one central sensor; others use a left and right pair, where the difference between what each sees is what places an object across the road rather than merely ahead.

It is the eye of adaptive cruise control, and on most vehicles of the automatic emergency braking and forward collision warning too — which is why a fault here can quietly remove the safety functions people rely on while the car drives perfectly normally.

Two failure modes dominate and neither is the sensor breaking. Obstruction: it sees through whatever is in front of it, so packed snow, ice, mud, road salt, a number plate or its surround, a bull bar or a metallic-painted aftermarket badge blind it completely, and a system reporting itself unavailable then is working correctly. Alignment: a fraction of a degree of aiming error becomes metres of lateral error at distance, so a parking knock leaving no visible mark can still matter. It cannot be aimed by eye — it needs the manufacturer's calibration procedure with a target board on a level floor, and that is required after any front-end work.