What does O/D OFF light mean?

The O/D OFF light indicates that the overdrive gear has been disengaged. This can happen for many reasons, such as if the vehicle is being driven in too high gear for the current speed or if the overdrive gear is damaged. The vehicle will typically lose some fuel efficiency when the overdrive gear is disengaged. In some cases, the loss in fuel efficiency may be minimal, but it can be significant in other cases. If the overdrive gear is disengaged frequently, it can lead to premature wear on the transmission and reduced fuel economy overall. For this reason, it's important to understand what causes the O/D OFF light to come on and to avoid driving in a way that will cause it to engage.