  We study transport properties in a slowly driven diffusive system where the
transport is externally controlled by a parameter $p$. Three types of behavior
are found: For $p<p'$ the system is not conducting at all. For intermediate $p$
a finite fraction of the external excitations propagate through the system.
Third, in the regime $p>p_c$ the system becomes completely conducting. For all
$p>p'$ the system exhibits self-organized critical behavior. In the middle of
this regime, at $p_c$, the system undergoes a continuous phase transition
described by critical exponents.
