Originally Posted by rmorrow

Look on the environment control display. When you press the button, it toggles "ECO" mode on that display. The effect is that heating and cooling functions are adjusted to minimize their impact on FE. Note that certain settings (like turning on front defrost, or setting temperature to MAX or MIN) will turn off ECO mode and the only way to turn it back on is to press the button again (after eliminating the setting that turned it off in the first place - if you leave defrost on, ECO can not be turned back on).