Finitely generated bimodules over Weyl algebras

Let $A$ be the $n$-th Weyl algebra over a field of characteristic zero, and $\phi :A\to A$ an endomorphism with $S=\phi (A)$. We prove that if $A$ is finitely generated as a left or right $S$-module, then $S=A$. The proof involves reduction to large positive characteristics. By holonomicity, $A$ is always finitely generated as an $S$-bimodule. Moreover, if this bimodule property could be transferred into a similar property in large positive characteristics, then we could again conclude that $A=S$. The latter would imply the Dixmier Conjecture.