  We consider a model of dark matter fluid based on a sector of Horndeski
gravity. The model is very successful, at the background level, in reproducing
the evolution of the Universe from early times to today. However, at the
perturbative level the model fails. To show this, we use the code
$\texttt{hi_class}$ and we compute the matter power spectrum and the cosmic
microwave background spectrum. Our results confirm, in a new and independent
way, that this sector of Horndeski gravity is not viable, in agreement with the
recent constraints coming from the measurement of the speed of gravitational
waves obtained from the observation of the neutron star merger event GW170817.
