Check that the two-wire rain sensor is correctly installed and is a normally closed rain sensor (Acqua Click model).
If the rain sensor is not present, check that the metal bridge in the terminal compartment at "SENS" is present.
In the case of an I-DIAL OUTDOOR control unit, check that the 220V mains connection has been made correctly (this operation requires a qualified electrician) and that an external 220/24 VAC transformer has not been installed.