Power Management software is active by default on sun4u systems (such as the Sun Ultra(TM) 1). Accordingly, the Power Management system for those machines is designed to evaluate each automatic power-cycle request that occurs due to the system being idle. If this evaluation determines that the thermal shock of power cycling on this occasion would contribute to decreasing hardware reliability, then this power-cycle request is deferred.
Despite this feature, you can still choose to turn off a sun4u system manually at any time. This checking feature is intended as a brake on the hardware being subjected to an excessive number of automatic power-cycle requests.