* The overdrive switch is turned off. The overdrive switch is typically located on the gearshift lever. When it is turned off, the transmission will not shift into overdrive, which is the highest gear. This is useful for driving in hilly or mountainous terrain, as it can help to prevent the transmission from shifting too frequently.
* The overdrive solenoid is malfunctioning. The overdrive solenoid is a small electrical device that controls the shifting of the transmission into overdrive. If the solenoid is malfunctioning, it may not be able to engage overdrive, which will cause the overdrive light to come on.
* There is a problem with the transmission. If there is a problem with the transmission, it may not be able to shift into overdrive. This can be caused by a variety of problems, such as low fluid levels, worn gears, or a faulty shift linkage.
If the overdrive light is on in your 2000 Ford Explorer, you should take it to a qualified mechanic to have the problem diagnosed and repaired.
