My bowflex 7 series treadmill keeps saying Err L5. Idk I…

The error is actually "LS" not L5 this is a low speed indicator. Here is how to check the speed sensor to see if it is working:
If you open the cover, you will see the main circuit board, and there will be a black wire that runs from that board to a sensor just beside the pulley that is on the end of the roller that turns the belt. Follow the instructions below to verify if the sensor is bad or not.

1) Align the magnet on the pulley to see if the LED  on the main board comes on. If it does not move the sensor until it is  about 1/8 of an inch from the magnet.

2) Unplug the sensor from the main board and test to see if the circuit open and closes when the magnet passes the sensor using a multi-meter.

3) Short the two wires in the plug to see if the LED comes on if the light comes on you know the board is working.

If your unit fails these tests the speed sensor needs replaced.

Did you check what I said?

Then try this

# get a 12-24 volt dc battery fully charged(like one from a cordless drill)
# Make sure it is fully charged, because it won't work if not
# attach the + and - to the motor red is + and black is -
# If the motor turns slowly your motor is ok, if not the motor is bad and would need replaced
# if the motor is ok then your motor controller(the board with the metal) is malfunctioning and would need replaced.

Sounds good