Ford Focus 2004 Owner's Manual page 115

A system failure will be indicated
by a deep continuous tone or by a
deep tone sounding in three-second
intervals. In both cases the system
is not reliable. Have the system
checked by an expert.
The measuring range is divided
into five warning zones. As the
distance to an obstacle reduces, a
warning sounds, beginning as a
slow bleep and increasing to a
continuous tone.
Always keep the sensors free from
dirt, ice and snow (do not clean
with sharp objects).
Driving
113