Overdrive not working?

[Rob Roy]

Actually what I see in run(before starting) is check engine in yellow, which my garage told me that it is yellow because of oxygen sensors. If you want to change it will cost you a lot, better leave alone.

That is simply false. The check engine light only has one color, and it illuminates when the key is turned on simply to demonstrate that the bulb is working. If there was an error code from the oxygen sensors, the light would be on while the car is running; and even if this was the case, the sensors are not very expensive and simple to replace.