Hi, thank you for contacting JustAnswer.com. My name is Russell. I will do my best to provide the right answer to your question.
That error code is, unfortunately, not specifically indicative. In general terms it could be
1) your treadmill belt needs lubricating and adjusting.
2) your speed sensor could be broken or more likely out of alignment and not registering properly
3) motor getting old / power supply not providing proper amount of power to motor that drives belt. Let's hope not, and try the first two first.
To address the first problem, follow the procedure below, and note that treadmill belt lubricant is not oil-based, is special stuff specifically made for treadmills. Here's the procedure to follow to properly adjust the walking belt:
The solution that you should try first, is that the belt might need readjusting and lubrication (with the special, non-grease, lubricant these treadmill belts generally require.)
The belt should be adjusted, generally speaking, so that it is loose enough that it does not quite slip, but nearly that loose.
If the belt is adjusted (or, has shrunk up with age, and lack of lubricant...) too tightly, then it can stop the treadmill from working or running. An over-tight treadmill belt can cause wear, or even damage, in many other ways, such as by overheating the motor.