Why is my laptop fan constantly running at full speed?

A fan running hard means the system is shedding heat, so the question is whether the heat is justified. There are two broad possibilities: something is genuinely working hard, or the cooling system has degraded and normal work now produces high temperatures.

Start with load. Open the system monitor and sort by processor usage. A single process consuming most of a core continuously is the answer, and the usual culprits are a browser tab running heavy script, a stalled update or indexing service, a synchronisation client re-scanning everything, or malware. Background indexing after a large file change is legitimate and finishes on its own; the others generally do not.

If nothing is obviously busy and the fan still runs, the cooling path is likely obstructed. Dust accumulates in the heatsink fins and forms a mat that air cannot pass through, which is by far the most common cause on any machine more than a year or two old. Blocked vents, use on soft surfaces such as beds and sofas, and elevated ambient temperature all compound it.

Thermal paste between the processor and heatsink dries over several years and conducts far less effectively once it does. Replacing it restores cooling substantially, though it requires disassembly.

Two software causes are worth checking before opening anything. Power settings configured for maximum performance keep the processor at high clocks unnecessarily, and an outdated firmware version can control fans poorly.

Sustained high temperature is worth addressing rather than tolerating, because the system responds by reducing performance to protect itself, so a noisy machine is usually also a slow one.