Make absolutely sure the harness connector going into the transmission is clean. It took two shop visits to get the mud and grease completely out of the socket on mine before the failsafe mode would stop. Same situation - undercarriage spray at the carwash, and then driving through a heavy rain the next time.

2002 X5 3.0 300,600 miles
2014 428i 17,000 miles

2004 325i sold at 123,600 miles
2001 325i sold at 66,000 miles