What you describe sounds somewhat like this. The CPU_FAN header on many mobos pays special attention to whether or not the fan on the CPU chip actually is turning and cooing it. It does this by monitoring the fan speed signal sent from that fan back to the CPU_FAN header on its Pin #3. In addition, the mobo monitors the CPU internal temperature (which is measured by a sensor inside the CPU and fed out on one of its pins). If the fan does not turn (that is, the mobo gets no speed signal on the CPU_FAN header), many will give out a short warning and then shut down completely before waiting for the CPU to overheat. Or, if the CPU does overheat after warming up from a "cold start", the system will shut down, too, In either case, what may...