Within the BIOS:
Buried inside the BIOS firmware is a lookup table that tells the fan: At 50°C, run at 30% speed; at 70°C, run at 60%; at 85°C, run at 100%. If this table is corrupted (rare but possible) or poorly designed (common in early BIOS versions), the fan will lag behind the temperature curve. toshiba dynabook bios hot