  We study scattering on an ultra-low potential in armchair graphene
nanoribbon. Using the continuous Dirac model and including a couple of
artificial waves in the scattering process, described by an augumented
scattering matrix, we derive a condition for the existence of a trapped mode.
We consider the threshold energies, where the the multiplicity of the
continuous spectrum changes and show that a trapped mode may appear for
energies slightly less than a threshold and its multiplicity does not exceed
one. For energies which are higher than a threshold, there are no trapped
modes, provided that the potential is sufficiently small.
