We study quantitative properties of solutions of operators of the type \({\mathcal L}= \sum_{j=1}^ p X_ j^ 2\), where \(X_ j\) are smooth vector fields in \(\mathbb{R}^ n\) satisfying Hörmander’s condition of hypoellipticity: rank Lie \([X_ 1,\dots, X_ p]=n\) at every \(x\in \mathbb{R}^ n\). Our main objective is to establish a uniform Harnack’s inequality for nonnegative solutions and the continuity of solutions of \((-{\mathcal L}+ V)u=0\), where \(V\) is a measurable function belonging to a suitable class.