The MC-2100 has a single test light. The meaning of the operation of the test light is:
If light it off - the board has no detectable input AC power.
If light is on and glowing solid - It is receiving AC power but no speed signal from the console.
If light is on and blinking rapidly - AC power is present, it is receiving a speed signal and is outputting DC voltage.
If light is on and blinking slowly around once per second - there is an amp problem with the treadmill. Usually this is a belt/deck friction problem.
If light is on and blinking very slowly at a rate of on one second and off one second - the controller is dropping output voltage by design. It does this if the maximum current limit is exceeded and increased amp load does not maintain belt speed. The reason it is dropping output is to prevent damage to the system. This is usually a bearing seizing up or a bad belt/deck friction problem.