If you're talking about a DEF tank level sensor, then
this is the link to check out. Some refer to it as the PIXLFIX fix!
On the other hand, the DEF tanks can also have the temperature sensor go bad. Not sure there is an "official" fix yet, but t
his posting from the M57 turbo diesel forum looks really promising.
Here is the link for a $4 NTC 10K thermistor. The work was done on a 335d rather than an e70 35d, but it should be very similar. The only thing I'm not sure of is whether the specs (10k ohms at 77 degrees F) match the original thermistor. Also not sure how the epoxy coating will hold up to urea.
Hope that helps.