The O/D Off light will turn on if you switch the overdrive function off. The O/D Off light means that your vehicle has disabled the overdrive functionality and hence your transmission cannot shift to the highest gear to facilitate acceleration on an upgrade or deceleration on a downgrade.
When the O/D Off light is on, you will be able to keep your vehicle in one gear so that you can have more manual control over your driving. This is useful in bad weather conditions when you need to be careful and precise with the maneuverability of your car.
Most automatic vehicles from Japan such as Toyota have the overdrive switch to help in switching between the usual four gears. In their cases, they won’t be able to engage the fourth gear when the overdrive is off but can be able to switch between the other three gears. You should make sure the O/D Off light is never on if you want your transmission to shift to the fourth gear to help with fuel economy.
The O/D Off light is useful when you want to help the onboard computer decide on which gear to select. This is usually the case when you are carrying a heavy load such as towing a trailer. In this case, the automatic transmission will try to shift back and forth between overdrive and the following highest gear to find a suitable speed.
Switching off the overdrive function will help your vehicle make a better decision regarding the driving speeds, engine revolutions, and gear selection. It is also favorable to let the O/D Off light stay on if you prefer engine braking when going downhill.
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