Also called: PAS sensor, pedal sensor, magnet disc
What it does
Once it sees the pedals turning, the motor gives the amount of help set by the assist level, however hard you push. It's simple and common on hub-motor bikes, and often uses a ring of magnets behind the crank.
Signs of a problem
- Assist that starts late or not at all
- Assist that keeps going briefly after you stop pedaling
Safe things to check
- Look at the magnet ring behind the crank: it should be in place, not cracked, and close to the sensor.
- Check the sensor's cable is plugged in.
Looking after it
- Keep the magnet ring clean; a damaged ring is a cheap part a shop can swap.