If you have the Classic trainer (the white one):
This is not indicative of a fault. In fact, in the Classic model this LED never lights up.
If your trainer model is the Pro (the black one), Pro 2 (the yellow one), Go or Air:
The LED must remain on in keeping with the color code of each model whenever the trainer is plugged in.
If you notice that the LED doesn't turn on, make sure you are using the power supply that was supplied with the trainer. Then, use a multimeter to verify that the voltage output by the power supply is between 12V and 15V.
If after doing these checks the LED on your trainer still doesn't turn on, please fill out the following form and we will contact you as soon as possible to give you a solution.
Please read the article "My trainer is not detected"