  We consider the generalized Yukawa model consisting of two Dirac fermions and
two scalar fields. One of the scalars is assumed to be much heavier than the
other particles, so it decouples at low energies. Low-energy effective
Lagrangian (EL) of the model is derived. It describes the contributions of the
heavy scalar into observables in the limit when this particle is decoupled. We
consider the cross-sections of $s$- and $t$-channel processes, obtained within
the complete model and its low-energy approximation. The contributions of the
one-loop corrections in the cross-sections coming from light particles are
analyzed. These are corrections to the parameters of the heavy boson and the
contributions of the one-loop mixing of light and heavy scalars. We identify
the ranges of Yukawa's couplings where the corrections are significant. We find
that if the interaction between fermions and either light or heavy scalar is
strong enough, the derived EL could not be applied for the description of the
analyzed cross-sections even if the heavy scalar decouples. Implications of our
results in searching for new particles beyond the Standard model are discussed.
