Something has happen to the transmission. Had the same problem when pulling a heavier trailer, after the road levelled out, the light would go out. Check Transmission at dealer.
If it has been switched off - the O / D - OFF light should light in the instrument cluster
Normally the overdrive is allowed , so the overdrive OFF light does not light up When the overdrive is switched off , the overdrive OFF light will light up to let you know that the overdrive has been switched off
Overdrive is the normally allowed position so you don't see the light on If you turn OFF the overdrive , the overdrive off light will light up or if there is a problem the overdrive off light will flash
You're screwed
The overdrive is "ON" when the light is "OFF". Notice the light for the overdrive has a slash through it.
If the overdrive's off light is on that means it's off. If it is off then the overdrive is on.
The overdrive light will be off when the overdrive switch is pushed in. If the light flashes then that's a trouble shooting code.
overdrive off light
The overdrive off light flashing - indicates a malfunction has been detected
keep the overdrive shaft button pressed, there is a panel light called "O/D" that should light indicating the overdrive is OFF. In that case make sure the light is OFF which means overdrive is ON.
If the light says off then your overdrive is turned off, push the button again and the light will turn off and overdrive will be on again.
That's the overdrive off light