Pedal assist not engaging — PAS sensor alignment guide

Pedal assist system (PAS) kicks in when the cranks turn past a threshold. If it stops working, the cause is almost always simple — a magnet ring has slipped out of alignment with the sensor, or the cable has come loose. 5 minutes to fix.

Quick diagnosis

Most of the time, it's one of these:

1. Magnet ring drifted out of sensor range — Cadence PAS: the magnet-ring-to-sensor gap must be ~2–3mm. Bumps shift the ring. Sensor sees no pulses → no assist.
2. PAS sensor cable disconnected or cut — Cable runs along BB to controller. Pedal heels can kick the connector off; pedalling cycle can fatigue-fail cable.
3. Controller PAS mode set to OFF — Some displays have PAS-off mode for walk-only use. Accidentally toggled.
4. Bosch/Shimano torque sensor drift — Rare. Torque sensor requires dealer re-calibration. Not home-fixable.

Fix flow

Step 1 — Check display PAS mode

• Is your assist level showing (P1–P5 or similar)? If showing '0' or 'OFF', toggle up.
• Walk-mode may be active (hold + button) — looks like PAS but only powers at ~6 km/h with bike moving.

Step 2 — Cadence sensor — inspect magnet ring alignment

1. Look at the non-drive side of the bottom bracket. You should see a circular magnet ring around the BB spindle.
2. A small sensor pod sits 2–3mm away from the ring, aligned with the magnets.
3. Common fault: ring has slid on the spindle, now sits too far from sensor. Or sensor bracket has bent/loosened.
4. Re-align: push ring along spindle to sit directly under sensor. Gap should be 2–3mm (thickness of a business card).
5. If ring is loose on the spindle, use a small amount of cyanoacrylate (superglue) or thread locker to re-secure.

Step 3 — Check the PAS cable

1. Follow PAS cable from sensor up to controller. Usually 3-pin JST-SM.
2. Inspect for cuts, pinch points (near crank arms), corrosion on connector pins.
3. Disconnect, clean pins, reconnect. Route cable away from pedal strike path.

Step 4 — Sensor voltage test

1. Multimeter on DC V 20V range.
2. Turn bike on. Connect probes to sensor signal output (middle wire, usually white or green) and ground.
3. Slowly rotate cranks — should see voltage pulse between ~0V and ~5V as magnets pass sensor.
4. No pulses → sensor dead, replace (£8–15 for budget sensor, £30–50 for OEM-style).

Step 5 — Replace a failed sensor

1. Most cadence sensors mount to chainstay with zip-ties or a bolt-on bracket.
2. New sensor installs same position. Ensure 2–3mm gap to magnet ring.
3. Connector is standard 3-pin JST-SM — match colours if unsure.
4. Test: lift bike, turn on, rotate cranks → motor should engage.

Torque-sensor ebikes (Bosch, Shimano, Yamaha)

These use an integrated torque sensor inside the drive unit — no user-accessible PAS ring. Faults require dealer diagnostic tool and firmware update. Budget £60–120 for dealer service, or warranty if under 2 years old.

Replacement parts