  We consider some (anisotropic and piecewise constant)
convection-diffusion-reaction problems in domains of R2, approximated by a
discontinuous Galerkin method with polynomials of any degree. We propose two a
posteriori error estimators based on gradient recovery by averaging. It is
shown that these estimators give rise to an upper bound where the constant is
explicitly known up to some additional terms that guarantees reliability. The
lower bound is also established, one being robust when the convection term (or
the reaction term) becomes dominant. Moreover, the estimator is asymptotically
exact when the recovered gradient is superconvergent. The reliability and
efficiency of the proposed estimators are confirmed by some numerical tests.
