The cursed PDC's sensors, I've replaced one or more in all my BMW's and did one on the sisters x1 last week.

I actually buy them on ebay, they are cheaper but you might have to colour code them yourself if there not on the plastic strip. Or BMW can get them them sprayed.

http://www.ebay.ie/itm/222053512752

That's the person I bought them from but don't know if they have the e39 version.

The pulse you hear from the working sensors is very faint, you'd want to put your ear over it and avoid a noisy area when checking.