The front fan cools the print cores during a print. This helps to prevent the heat from the nozzle traveling too far upwards.
The fan takes in air from the front of the print head and directs it towards the print cores. Sometimes the airflow causes thin strands of filament to be sucked into the fan during a print. If strands accumulate in the fan, they can obstruct the fan and prevent it from spinning.
To check the front fan, first gently open the front fan bracket. Blow into the front fan to see if it spins smoothly. If it does not move at all, or stops spinning abruptly, carefully clear any obstructions from the front fan with some tweezers.
Caution: Make sure that the print cores have completely cooled and the printer is turned off before performing this check.
If the fan is still not spinning after removing visible filament residue, it should be replaced.