We study the limit of the hyperbolic–parabolic approximation $$\left\{\begin{array}{l@{\quad}l@{\quad}l} v^{\varepsilon}_t + \tilde{A} \left(v^{\varepsilon}, \, \varepsilon v^{\varepsilon}_x \right) v^{\varepsilon}_x = \varepsilon \tilde{B}(v^{\varepsilon} ) v^{\varepsilon}_{xx} \quad v^{\varepsilon} \in \mathbb{R}^N \cr \tilde{\rm \ss}(v^{\varepsilon} (t, \, 0)) \equiv \bar g \cr v^{\varepsilon} (0, \, x) \equiv \bar{v}_0. \end{array} \right.$$ The function $${\tilde {\ss}}$$ is defined in such a way as to guarantee that the initial boundary value problem is well posed even if $${\tilde {B}}$$ is not invertible. The data $${\bar {g}}$$ and $${\bar {v}_{0}}$$ are constant. When $${\tilde {B}}$$ is invertible, the previous problem takes the simpler form $$\left\{\begin{array}{l@{\quad}l@{\quad}l} v^{\varepsilon}_t + \tilde{A}\left(v^{\varepsilon}, \, \varepsilon v^{\varepsilon}_x \right) v^{\varepsilon}_x = \varepsilon \tilde{B}(v^{\varepsilon} ) v^{\varepsilon}_{xx}\quad v^{\varepsilon} \in \mathbb{R}^N \cr v^{\varepsilon} (t, \, 0) \equiv \bar v_b \cr v^{\varepsilon} (0, \, x) \equiv \bar{v}_0. \end{array} \right.$$ Again, the data $${\bar {v}_b}$$ and $${\bar {v}_0}$$ are constant. The conservative case is included in the previous formulations. Convergence of the $${v^{\varepsilon}}$$ , smallness of the total variation and other technical hypotheses are assumed, and a complete characterization of the limit is provided. The most interesting points are the following: First, the boundary characteristic case is considered, that is, one eigenvalue of $${\tilde {A}}$$ can be 0. Second, as pointed out before, we take into account the possibility that $${\tilde {B}}$$ is not invertible. To deal with this case, we take as hypotheses conditions that were introduced by Kawashima and Shizuta relying on physically meaningful examples. We also introduce a new condition of block linear degeneracy. We prove that, if this condition is not satisfied, then pathological behaviors may occur.