When a laptop operates, the processor circuitry, battery and hard drive all generate tremendous amounts of heat. Sensitive components inside the laptop are susceptible to heat-related damage, making evacuation of hot air from the laptop's casing critical to safe operation. When temperature sensors inside a laptop computer identify increasing amounts of heat, the fan is activated and controlled to route heat out of and away from the machine.
Inhibited Function of the Fan May Cause it to Run
Since the primary function of a fan is to evacuate excess heat from a laptop, and its operation is generally controlled by one or more heat sensors, the inability of the fan to effectively remove hot air may cause it to spin up to--and remain at--full speed. One common cause for this issue is debris that clogs the laptop's air vents. In a dusty or pet hair-laden environment, dust or dander can accumulate around the laptop air vents and block the exit of hot air. Some users who place their laptop computers on fabric, foam or other surfaces that conform to the machine's shape can also inadvertently block the air vents. In such cases, the fan may run continuously in an attempt to force the hot air out of the machine. Simply clearing the air vents can resolve the issue and allow the fan to return to normal operation.
Hardware Issues May Cause the Fan to Run Continuously
If all debris blocking the air vents is removed and the computer is operating in a reasonably cool environment, the fan could be running continuously due to faulty wiring or improper installation. Many laptop fans are specifically designed to route air in one specific direction (the direction of the air vents) to most effectively remove heat. If the fan has been recently removed and replaced, it may have been incorrectly installed; an incorrectly installed cooling fan simply routes hot air around the inside of the laptop case instead of removing it. Since the hot air is never removed, the fan runs continuously until the laptop eventually overheats. In addition, failed or faulty temperature sensors can incorrectly alert the computer to a high temperature condition; the computer then spins the fan up to full speed in an attempt to remove the hot air. Finally, some users report removing a laptop cooling fan and reattaching the fan's power wires in incorrect polarity; such a wiring issue may cause the fan to malfunction and run continuously.