The sensors themselves are the most common cause. They go bad. Cost in
the US is something like $120/each (highway robbery - similar kits -
using the same type - but of course not identical ultrasonic sensors -
go for $30 on Ebay every day - with 6 sensors..)

Problem is identifying which sensor is bad. To do that - you'll need to
visit the dealer, or find a local garage with an AutoLogic code scanning
tool (which is made in the UK - so hopefully you can find one..)  That
will pin down which sensor is bad. Replacement is simple - pull the trim
strip on the bumper, pull the sensor out, unplug it, plug new one in,
replace trim strip.

My '03 is heading to the dealer tonight for the same problem - will be
the 3rd sensor that has needed replacement. Luckily I have an extended
BMW CPO warranty on the car that covers this.

Mike

I know this is old but when mine stopped the local dealer wanted to charge me
for a new control unit that didn't fix the fault.  After a week of messing me
about and then not touching the car when I took it in for a third time I created
merry HELL!

they loaned me an M3 until they fixed my PDC and after 4 days they (or someone)
found it was a relay (can't remember where but probably either in the boot or
bonnet area) (I have the BMW DVD/CD electrical disc but it doesn't show exactly
where either)

Look for a RELAY.

And remember my advice when going to a garage or fix-it place -

1    Always as if they CAN fix it. - Do NOT move from this stance keep saying
- whatever they say or technical words they use - stick with "CAN YOU
    FIX IT" and don't accept anything else.  If they can't get them to say
    so or if they won't say they CAN fix it go elsewhere.

2    NEVER tell them what is wrong (you are not an expert - even if you are)

3    NEVER sign anything that indicates exactly what they will be doing eg.
Change PDC control unit.  It might not be the PDC control unit but as
    you have agreed to that being done you MIGHT have to pay for it.

If they do NOT fix it then you DO NOT PAY.

That is why BMW dealers always say "It's £40 to diagnose your OBC then we will
tell you what's wrong"

Unfortunately it's a sign of these computerised and highly technical cars we
drive these days.