In my car, I can feel something kicking in and out separate from the ECO light, with the ECO light still on. It feels pretty much like a transmission shift, but isn't, because the tachometer is unaffected. At cruising speed, anytime I push on the accelerator, even seemingly 1/16", I get this "shift." ECO remains lit. (If I push on the gas a little more, ECO goes out, and there's no percepible mechanical change at this point.) I strongly suspect it's the 3 cyl. mode kicking in, and it's not associated with the ECO light. More evidence is that at some speeds, like about 52 miles per hour, there's a noticeable amount of what seems like engine vibration that appears when letting off the accelerator a little and prompting this "shift." The car is supposed to have engine mounts that help eliminate vibration, but in my car at least, I can easily feel it.

My "lifetime" mpg over 6500 miles is over 35 mpg, so I don't think my ECO/3-cyl system is defective. (Maybe the engine mounts have a problem, and/or my car is "rough" when engaging 3-cyl mode, as others report they can't feel it.)