Let C(λ) ⊂ [0, 1] denote the central Cantor set generated by a sequence λ = (λn) ∈ 0, 1 2 N . By the known trichotomy, the difference set C(λ) − C(λ) of C(λ) is one of three possible sets: a finite union of closed intervals, a Cantor set, and a Cantorval. Our main result describes effective conditions for (λn) which guarantee that C(λ) − C(λ) is a Cantorval. We show that these conditions can be expressed in several equivalent forms. Under additional assumptions, the measure of the Cantorval C(λ) − C(λ) is established. We give an application of the proved theorems for the achievement sets of some fast convergent series.