If the fluid level is correct, first try cleaning the sensor contacts with contact cleaner. If this does not work, replace the sensor. If this does not work, drink two beers and call me in the morning.
1996 (W210) E300D 86K - Traded in for a Lexus
1992 (W124) 300D 2.5 Turbo 202K - Sold
1983 (W123) 300D, 146K - Sold
1970 280S, 263K - Sold - Beginning of addiction