I have a 2001 E320 with about 41,000 miles on it. While driving it in the rain in New York yesterday, the "Display Defective" light came on. It's the indicator that looks like an engine with the propeller on the left. I looked in the manual and it says:

"This message is displayed to indicate that the information being relayed by the engine control unit is no longer complete. The coolant temperature gauge, tachometer, or the cruise control may have failed."

When I got home, I realized that the car would only stay in first gear and won't shift into second gear when I reach 30 mph. It just sort of hums and won't shift. Other than that, the car runs fine and the engine is still purring smoothly whenever I turn the ignition.

My question is, in cases like this, does the engine control unit need to be replaced or is it something more ominous?