  We consider subelliptic equations in non divergence form of the type $Lu =
\sum a_{ij} X_jX_iu=0$, where $X_j$ are the Grushin vector fields, and the
matrix coefficient is uniformly elliptic. We obtain a scale invariant Harnack's
inequality on the $X_j$'s CC balls for nonnegative solutions under the only
assumption that the ratio between the maximum and minimum eigenvalues of the
coefficient matrix is bounded. In the paper we first prove a weighted
Aleksandrov Bakelman Pucci estimate, and then we show a critical density
estimate, the double ball property and the power decay property. Once this is
established, Harnack's inequality follows directly from the axiomatic theory
developed by Di Fazio, Gutierrez and Lanconelli in [6].
