Volvo 2008 V70 Owner's Manual page 153

Accessories or other objects must not be installed in front of the grille.
Modification of the radar sensor could make its use illegal.
The radar sensor's capacity to detect vehicles ahead is impeded:
if the radar sensor is obstructed and cannot detect other vehicles, for example in heavy rain, or if snow or other
objects are obscuring the radar sensor.
NOTE
Keep the area in front of the radar sensor clean.
if the speed of vehicles ahead is significantly different from your own speed.
The radar sensor has a limited field of vision. In some situations it may detect a vehicle later than expected or not
detect vehicles at all.
In certain situations, the radar sensor cannot detect vehicles at close quarters, for example a vehicle that suddenly
enters the lanes between your vehicle and the one that the system has already detected.
Small vehicles, such as motorcycles, or vehicles not driving in the center of the lane may remain undetected.
In curves, the radar sensor may detect the wrong vehicle or lose a detected vehicle from view.
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04 Comfort and driving pleasure
Adaptive Cruise Control-ACC
Fault tracing and actions
If the message Radar blocked See manual
obstructed and that a vehicle ahead cannot be detected.
is displayed, this means that the radar signals from the sensor have been