When an e-bike motor turns on but fails to provide power while pedaling, the issue is almost always a breakdown in the Pedal Assist System (PAS) sensing loop. This usually happens when a cadence magnet disc slips on the bottom bracket spindle, a brake cutoff switch gets stuck engaged, or the PAS sensor wire disconnects. Because electrical power remains active, riders often lose assistance without seeing an error code on the display.
Fast-Fix: The 45-Second Solution
When the display lights up but pedaling provides no motor power, the system faces a low-safety-risk riding inconvenience. First, confirm the display’s PAS level is set above 0. Then, check the bottom bracket to ensure the magnet disc spins within 2 to 3 millimeters of the sensor without slipping, and verify that both brake levers are completely released so their motor cutoff switches aren’t engaged.
Quick Risk Snapshot
- Severity Tier: Low to Moderate (Electrical Sensing Fault)
- Safe to Ride? Safe to ride under manual pedal power or throttle; electric pedal assistance is non-responsive.
- Most Common Cause: Misaligned cadence magnet disc, dirty pickup sensor, or a gap greater than 3 millimeters between disc and sensor.
- Rare but Serious Cause: Blown controller sensor input port or snapped internal torque transducer wiring.
When This Is Low Risk vs High Risk
- If throttle works fine but pedaling gives zero response: This is lower risk. The motor, battery, and controller power stages are healthy; the fault is isolated strictly to the pedal assist input signal or brake sensor feed.
- If neither throttle nor pedal assist work, but the display stays on: Escalate immediately. The issue is likely a latched brake cutoff switch, tripped controller protection, or main harness fault. See E-Bike Display Lights Up But Motor Won’t Start.
- If pedaling produces stuttering motor power accompanied by burning smells or error codes: Shut off power immediately. The motor or controller is suffering an electrical phase short or severe sensor cross-talk.
What This Usually Means
To deliver power when you pedal, your e-bike controller relies on a continuous stream of electrical signals from either a cadence sensor or a torque sensor located near the bottom bracket.
Cadence sensors use a plastic ring containing 8 to 12 small magnets that rotate past a stationary Hall sensor pickup mounted to the frame. As you pedal, the spinning magnets pass the pickup, creating rapid high/low voltage pulses. When the controller receives these pulses, it turns on the motor.
Think of a basic cadence sensor like a lighthouse keeper looking at a turning waterwheel. If the waterwheel turns, the lighthouse keeper flips a switch to turn on the main pump. If the waterwheel stops turning, or if the wheel breaks off its axle and spins loosely, no signal is seen, and the pump stays off.
If your motor fails to engage while pedaling, the controller is either not receiving those voltage pulses or is actively ignoring them. A slipping magnet disc, a wide sensor gap, or a stuck brake cutoff switch acts like a broken signal wire, leaving the controller unaware that you are turning the cranks.
Probability Breakdown
| Failure Point | Confidence Range | Physical Cause |
|---|---|---|
| Misaligned / Slipping Cadence Magnet Disc | 60% – 70% | The plastic magnet ring on the bottom bracket spindle has pushed away from the pickup sensor or spins freely without turning with the crank arm. |
| Stuck Brake Cutoff Sensor | 20% – 30% | The reed switch or Hall sensor inside a brake lever mechanism is stuck triggered, continuously telling the controller to disengage the motor. |
| Damaged PAS Wiring or Corroded Connector | 5% – 10% | Water ingress, road debris, or chain rub has damaged the 3-pin Julet connector or severed the signal wire near the chainstay. |
| Internal Torque Sensor or Controller Logic Fault | 1% – 5% | Broken strain gauge, failed Hall element inside a sealed bottom bracket cartridge, or a dead sensor input pin on the controller board. |
What Increases the Risk
- Riding Through Deep Mud or Wet Gravel: Grime builds up on the face of the magnet disc, blocking magnetic pulses or pushing the plastic ring off the spindle.
- Car Rack Transportation: Road vibration during highway transport can shift fragile plastic disc rings or snag exposed bottom bracket wiring.
- Bumping or Dropping Brake Levers: Impact can dislodge small internal brake cutoff magnets, locking the sensor in an “always braking” state.
- Winter Road Salt: Salt water corrodes unsealed 3-pin harness connectors near the bottom bracket, interrupting low-voltage sensor signals.
If Ignored: 24 Hours → 1 Week → 1 Month
[24 Hours] ➔ Bike remains rideable under human power, but loss of pedal assist causes rider fatigue and heavy reliance on the throttle.
[1 Week] ➔ A loose magnet disc rubbing against the bottom bracket shell strips its plastic spline teeth, requiring a full ring replacement.
[1 Month] ➔ Unresolved moisture inside unsealed PAS cable connectors causes pin corrosion, requiring harness or controller repair.
What This Is Often Confused With
- Throttle Assembly Failure: If pedal assist works normally but the thumb or twist throttle does not respond, the problem is in the throttle circuit rather than the PAS system. See How to Fix an E-Bike Throttle That Isn’t Working and E-Bike Throttle Sensor Failure: Symptoms and Fixes.
- PAS Cutout Under Load: If pedal assist engages initially on flat ground but cuts out on steep hills, the battery BMS or controller is tripping from over-current, not a dead sensor. See Why Your E-Bike Pedal Assist Cuts Out While Riding.
- Sluggish Assist vs. Total Loss: If assistance is present but feels weak, delayed, or erratic, the sensor is sending signals but needs cleaning or recalibration. See Troubleshooting E-Bike Pedal Assist (PAS) Sensor Failures and E-Bike Torque Sensor Failure: Sluggish or No Assistance.
What To Do Right Now
- Verify Display Settings: Ensure your display power is on and the assist level is set to level 1 or higher (level 0 completely disables pedal assistance).
- Inspect the Magnet Disc: Look at the plastic ring mounted on the bottom bracket axle near the left crank arm. Push it firmly toward the frame until the gap between the magnet ring and stationary pickup sensor is 1 to 2 millimeters.
- Clean the Sensor Face: Wipe away dirt, metal shavings, or caked grease from the pickup sensor face and magnet disc using a clean rag and isopropyl alcohol.
- Check Brake Levers: Push both brake levers firmly forward to make sure they are fully returning to their rest position. If a lever plunger sticks, it will trigger the motor cutoff.
- Reseat the Connector: Trace the PAS cable to its inline 3-pin connector near the chainstay. Firmly press the waterproof connector together, ensuring alignment arrows point directly at each other.
When To Stop Immediately
- The motor engages in violent, sudden bursts when you are NOT pedaling (“ghost pedaling”).
- You notice smoke, a hot plastic smell, or melted wire insulation near the controller.
- Frayed or bare copper wires are visible near the bottom bracket shell or chainstay.
- The display shows active controller hardware or motor phase error codes.
What a Professional Will Check
- Multi-Meter Voltage Signal Test: Back-probe the PAS connector signal wire (typically blue or green) with a multimeter set to DC voltage. Rotate the pedals by hand to confirm voltage pulses cleanly between 0V and 5V.
- Brake Cutoff Isolation: Unplug the brake cutoff sensors one at a time at the handlebar or main harness to determine if a faulty brake lever switch is blocking the controller signal.
- Display Parameter Audit: Enter the display settings menu to verify parameter values (such as C1 or P-parameters) match factory specifications for sensor pole count and orientation.
- Torque Sensor Signal Measurement: On torque-sensing systems, measure baseline output voltage across the strain gauge pins to check for broken internal sensor wires.
Typical Repair Range
- Minor Repair (DIY Alignment / Magnet Disc Replacement): $0 – $20 for realigning existing parts or buying a replacement split-ring cadence disc.
- Moderate Repair (PAS Sensor / Brake Cutoff Replacement): $30 – $80 for a new sealed bottom bracket cadence sensor or replacement brake lever cut-off switch.
- Major Repair (Controller or Integrated Torque Bottom Bracket): $120 – $250+ if a shorted sensor damaged the controller logic board or the bike uses an integrated strain-gauge bottom bracket cartridge.
Related Symptom Escalators
- If your throttle works normally but pedaling provides no power: E-Bike Throttle Works But Pedal Assist (PAS) Does Not
- Troubleshooting broader pedal assist failures and delays: Troubleshooting E-Bike Pedal Assist (PAS) Failures and Delays
- Hardware replacement guide for dead cadence sensors: E-Bike Cadence Sensor Failure: Pedal Assist Not Engaging
- Diagnosing brake cutoff switches preventing motor start: E-Bike Motor Won’t Start After Braking? Check the Sensors
Ride Check
A pedal assist failure is usually a simple signal gap rather than a dead motor or blown battery. By pushing your magnet ring back into place, cleaning off road grime, and ensuring your brake levers return fully forward, you can resolve the majority of PAS issues right in your driveway. If physical adjustments do not restore power, testing cable continuity and replacing the inexpensive sensor pickup will get your e-bike assisting smoothly again.