ECO is designed to use an "economy" version of the heating and ac. Basically it helps save gas and has been on most Toyota Carmies (and other models I am sure) for over 20 years.

When you turn it on, there is a green light that should be lit within the button and the heat/ac display should show the letters ECO in the middle-left.

To maximize fuel economy, use the ECO button at all times. To get faster heating and cooling, turn off ECO.

Note: If you use the defroster, ECO will ALWAYS be OFF.

Mike