Without knowing if there are shift issues, it's difficult to know for sure. There isn't a overdrive sensor per say. The flashing light is diagnosed by pulling trouble codes from the PCM (powertrain control module) Most of those I recall were due to band failure internally. I would advise the issue be properly diagnosed though given it could result in considerable expense if wrong. If the transmission seems to be soft, mushy or lazy in it's shift patterns or very abrupt especially 1-2 and 3-4 up or down, it also points to a overdrive band problem or similar.