This Diagnostics version of the Fan SSR has two buttons and two LEDs. The buttons simulate the temperature switch in the radiator by momentarily activating low-speed or high-speed fan operation. By pressing the button for low-speed fans, followed by high-speed fans (Ignition Switch must be in the "ON" position), you can hear the fan speed increase. Simply verify the sound and that both fans are spinning, and you have instantly verified the relay, fuses, fans, resistors (low speed operation), and associated wiring are all functioning. Using the LEDs, you can monitor if the temp switch in the radiator is commanding low-speed and/or highspeed fans. When the temp switch grounds the temp "slow" or "fast" control line to activate low-speed or high-speed fans, the corresponding LED will illuminate.