Eco-Mode (battery economy) seems to be a French peculiar and nobody seems to have anything good to say about it - it's as if the vehicle is self-harming.
This below from a reply on here in December 2012...
C3 in Eco mode unable to lock doors or start car.
Followed Mick K advice above.
Disconnect battery Pos when ignition set on. Randomly press the central lock and speed limiter buttons on central column untill Eco mode suddenly went. Car started first time.
Eco mode what a stupid idea ?
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Will very probably lock the radio until code re-entered.
Good luck..., January 2015
