Generally, the L5 error indicates that the controller circuit is not receiving any data feed from the speed sensor of your treadmill.
When you start the treadmill (that has a L5 error), it will start briefly and then stop. It stops due to a built-in safety feature i.e. no speed data = stop motor.
To fix this issue:
Remove the motor cover, the speed sensor is located near the small wheel that has the small belt that connects with the roller. The sensor is a little (usually black) tube like thing.
In 90% of the cases, the sensor position has shifted (usually happens if you're not too careful when removing or fitting the motor cover). The sensor position is sensitive with respect to the two magnets embedded amongst the spokes of the small wheel (see photo).
In some treadmills, the sensor is fitted parallel to the wheel while in others, it is fitted perpendicular to the wheel.
All you have to do is move the sensor a tiny bit to the front (or back) and restart the treadmill. If this did not work try to reposition again (see attached photo). Depending on how far the sensor shifted from its correct position, it might take a few tries before you get it right. You know you've got it right when the treadmill once again functions normally.
In 10% of the cases, the sensor wire might have either got disconnected or broken (usually happens if you're not too careful when removing or fitting the motor cover). If you are a DIY kind of person and have a soldering kit handy, then you will know what to do else, call the technician - shouldn't take more than 15 minutes to fix it.
Hope this helped.