Cooling fans: signs of failure and how to find the right replacement

What a cooling fan does

A cooling fan moves air across the fins of a heatsink, which carries heat away from the processor, the graphics chip and other hot parts. In a laptop it is usually a flat blower fan that pushes air through a small heatsink connected to the chips by copper heat pipes, and the fan and heatsink are often sold together as one thermal module. A desktop usually has a fan on the processor cooler, one or more case fans and a fan inside the power supply. A server has a row of fan modules, often hot-pluggable, that the system watches and speeds up or down as it needs.

Signs the fan needs replacing

• It grinds, clicks, rattles or whines, especially at start-up. Worn bearings tend to get louder over time.
• It runs at full speed all the time, even when the machine is doing very little.
• The machine gets hot to the touch, slows down under load, or switches itself off without warning.
• The machine shows a fan error at start-up. Some laptops refuse to continue until the fan is replaced.
• The fan does not spin at all, or a server's management console reports a failed or missing fan.

Clean it before you replace it

Dust packed into the heatsink fins is a very common cause of a loud, hot laptop, and the fan itself may be fine. With the machine off and unplugged, blow compressed air through the vents or, with the cover off, across the fins. If you can reach the fan, hold the blades still with a finger or a plastic tool while you do it. Use short bursts, and keep the can upright. Spinning a fan fast with compressed air can damage its bearing. If the noise and heat return once it is clean, the fan is worn.

Laptop fans and thermal modules

• Check whether you need the fan alone or the fan with its heatsink. The name usually says fan, heatsink, or thermal module for the two together. HP part names often say heatsink or fan and heatsink assembly; Lenovo and Dell often say thermal module or heatsink assembly.
• Versions of the same laptop model with and without a separate graphics chip often use different heatsinks and fans. The part number or the machine's serial number tells them apart.
• Laptops with two fans often use a different part for the left and the right fan.
• When the heatsink comes off the processor, clean off the old thermal paste and apply a thin, fresh layer before refitting it.
• Thermal pads that are still whole can be reused. Replace any that tear with pads of the same thickness.
• A new module that comes with paste already applied is fitted as it is.

Desktop and server fans

• Standard case fans are sized by the frame width in millimetres, commonly 80, 92, 120 or 140 mm, and by thickness, commonly 25 mm. Most run on 12 volts DC.
• A 2-pin fan only takes power.
• A 3-pin fan also reports its speed, and the system can slow it only by lowering the voltage.
• A 4-pin (PWM) fan adds a control wire so the system sets its speed precisely.
• A 4-pin fan runs on a 3-pin header and the other way round, but speed control may be lost, and a fan that reports no speed can cause a fan error.
• Branded office desktops and small-form-factor models often use their own connectors and cable lengths, so match the part number rather than the size alone.
• Server fans are made for one server model or generation and slot into a fan bay. Many servers warn when a bay is empty, and some reduce performance or shut down if a fan is missing.
• A fan inside a power supply is part of the power supply. Replace the whole power supply rather than opening it, because its capacitors can hold a dangerous charge for some time after it is unplugged.

How to choose the right fan

Use these checks in order:

• Read the part number on the fan's label, usually on the hub or the frame. Laptop makers print their own spare number there, such as an HP spare number, a Lenovo FRU number or a Dell part number.
• If you cannot get to the fan, use the machine's exact model or product number and its serial number. The serial number identifies which version of the model you have.
• Compare the connector, the cable length and the position of the screw holes with the old fan.

Fitting a fan safely

• Shut the machine down fully and unplug it. In a laptop, remove the battery if it is removable. If it is internal, disconnect it as soon as the bottom cover is off, before touching anything else.
• Touch bare metal before handling parts.
• Keep track of which screw came from which hole, because lengths can differ.
• Route the fan cable away from the blades and plug it into the same header as before.
• Start the machine and check that the fan spins and the temperatures settle under load.