How to Fix an E-Bike Pedal Assist Sensor

This is the most common fix and the first thing I check when the assistance stops working.

Find the magnet ring. On most e-Bikes, it is a plastic ring with small magnets embedded, mounted on the left crank arm or around the bottom bracket. It sits right next to a small sensor attached to the frame. The gap between the ring and the sensor should be small, typically around 1 to 3mm, but the exact figure depends on your bike.

Look at both components. Is the magnet ring still in its correct position? If it has moved, rotate it back so the magnets are evenly spaced and pass directly in front of the sensor as you pedal.

Check the sensor mounting too. If the sensor bracket has bent or the bolt has worked loose, the sensor may have moved away from the ring. The gap between the two is critical for the assistance to work correctly. If they are too far apart, the sensor can’t detect the magnets’ signal. In this case, push the sensor gently back toward the ring and tighten the mounting bolt if it has come loose.

Once you’ve made any adjustments, give the crank a spin by hand and watch the sensor (be careful not to hit your fingers). You should be able to see the magnets passing the sensor face. Some sensors have a small indicator light that flashes each time a magnet passes, making this much easier to check.