I don't know about the, "if you get a legitimate alarm on a high mileage
car, its probably
too late" notion.  I got one a few weeks ago on my 87 Westy and  checked it
out.  No
problem.  Drove a couple more days and got the same alarm.  Took  it to my
mechanic
where he replaced both sensors and I haven't had a problem since.   Seems to
me that
they are fairly sensitive, and will even let you know when they are going
out ;-)  (Btw, I
guestimate that my mileage is around 150,000).
my $0.02 worth,
James Benthall
Houston, Texas
87 Westy (The Phoenix)