The auxiliary fan is an electric fan mounted in front of the radiator and condenser. The auxiliary fan is turned on by temperature sensors when certain pressure thresholds are reached in the A/C system, and provides additional cooling to bring pressures down below these levels. If the auxiliary fan system fails (fan never comes on) it causes the operating pressures to remain high and adds extra heat and stress on the A/C compressor, which can lead to premature failure. To test the auxiliary fan, first check the fuses, and if good, see if there is power to the fan itself. If you need to replace the sometimes hard-to-find auxiliary fan screen, check out our selection; we probably have the one you need.