Transmission problem
2002 Ford Escape 6 cyl Two Wheel Drive Automatic 114,70 miles
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A few minutes after starting my car, the O/D light begins flashing. I took it to a mechanic and he said a sensor in the transmission is malfunctioning. The Overdrive works; and if I turn it on when I first start the car, the light comes on like it is supposed to. However, after a couple of minutes it will start flashing again.

Is the mechanic right, or should I be worried about a bigger problem?