When an e-bike throttle accelerates the bike normally but pedaling provides zero motor assistance, the motor, battery, and main controller power circuits are working properly. The failure is strictly isolated to the pedal assist sensing loop. In most cases, this is caused by a magnetic cadence disc that has slipped along the bottom bracket axle, a dirty or misaligned pickup sensor, an accidental display setting change (PAS level 0), or a disconnected sensor wire. Because power reaches the motor via the throttle, the system is fully functional except for pedal input detection.
Fast-Fix: The 45-Second Solution
When throttle works but pedal assist fails, the issue is an isolated electrical sensing fault. First, verify the display’s PAS level is set above 0. Then, inspect the bottom bracket cadence disc to ensure it rotates alongside the crank arm and maintains a precise 1 to 2 millimeter gap from the pickup sensor.
Quick Risk Snapshot
- Severity Tier: Low (Sensing system failure; motor power remains accessible via throttle).
- Safe to Ride? Safe to ride using pedal power manually or accelerating via the throttle.
- Most Common Cause: Loose magnet disc, wide sensor gap (>3mm), or dirty pickup head at the bottom bracket.
- Rare but Serious Cause: Blown controller sensor input circuit or snapped internal torque transducer strain gauge.
When This Is Low Risk vs High Risk
- If the throttle drives the bike smoothly at full power and the display reads normal: This is lower risk. The high-voltage battery path, controller MOSFETs, and motor phase lines are fully intact. The issue is purely a low-voltage sensor signal breakdown.
- If pedaling causes error codes to flash on the display or produces sudden motor power surges: Escalate immediately. The sensor may be sending unstable voltage spikes or cross-talk signals to the controller.
- If neither throttle nor pedal assist work when pedaling, or the motor produces grinding sounds: Shut off power immediately. This indicates a mechanical drive disconnection or total controller shutoff rather than a PAS-specific sensor issue.
What This Usually Means
Electric pedal assist relies on a dedicated sensor at the bottom bracket to tell the controller when your feet are turning the cranks. On most budget and mid-range e-bikes, this is a cadence sensor consisting of a plastic ring embedded with 8 to 12 small magnets and a stationary Hall effect sensor. When you pedal, the spinning magnets pass the sensor, generating rapid low-voltage pulses (0V to 5V).
Think of a cadence PAS sensor like a revolving turnstile at an amusement park. Each time a magnet passes the sensor, it clicks the turnstile gate. If the turnstile clicks rapidly, the controller turns on the motor. If the magnet ring slides down the axle away from the sensor, or if the ring cracks and stops spinning when the crank turns, the turnstile never clicks. The controller sits waiting for input that never arrives, even though the motor itself works fine when bypassed by the throttle button.
On premium e-bikes equipped with a torque sensor, the system measures physical pedal force via a strain gauge rather than just rotation speed. If the internal strain gauge wires fatigue or lose calibration, the controller receives a zero-force reading regardless of how hard you pedal.
Probability Breakdown
| Failure Point | Confidence Range | Physical Cause |
|---|---|---|
| Slipping or Misaligned Magnet Disc | 55% – 65% | Plastic cadence ring shifted down the bottom bracket spindle, expanding the sensor gap beyond 3mm or spinning loosely on the axle. |
| Disconnected or Damaged PAS Cable | 20% – 25% | Water ingress, chain slap, or road debris pulled or corroded the 3-pin Julet connector near the chainstay. |
| Display Parameter / PAS Level 0 | 10% – 15% | Handlebar display set to Level 0 assist, or internal controller setting (like PAS magnet count parameter C1) misconfigured. |
| Faulty PAS Sensor or Controller Board | 5% – 10% | Internal Hall chip inside the pickup failed, or the controller’s 5V sensor supply pin burned out internally. |
What Increases the Risk
- Riding Through Heavy Rain or Mud: Water and road grime build up on the face of the magnet disc, blocking magnetic signals or corroding the 3-pin cable plug.
- Chain Drops or Rough Trail Riding: A dropped chain can slap the PAS cable near the bottom bracket shell, cutting or pulling signal wires.
- Transporting Bike on Car Racks: Road turbulence can shift loose plastic magnet discs along the bottom bracket axle.
- Aftermarket Crank Set Installation: Replacing crank arms without checking spindle spacing often pushes the magnet ring out of alignment with the pickup sensor.
If Ignored: 24 Hours → 1 Week → 1 Month
[24 Hours] ➔ Bike remains rideable using throttle power, but battery range drops due to lack of rider pedaling contribution.
[1 Week] ➔ A loose magnet disc rubbing against the bottom bracket shell wears out its mounting splines, requiring a full ring replacement.
[1 Month] ➔ Moisture trapped in an unsealed PAS wiring connector causes pin corrosion, requiring harness splicing or controller replacement.
What This Is Often Confused With
- General PAS Failure (No Throttle Available): If neither pedaling nor the throttle powers the motor, the problem is a blown fuse, tripped brake cutoff, or dead battery line, not just a PAS sensor fault. See E-Bike Display Lights Up But Motor Won’t Start and E-Bike Motor Not Working When Pedaling (PAS Troubleshooting).
- Brake Cutoff Switch Latch: If a brake lever switch sticks open, it disables both throttle and pedal assist simultaneously on most controllers. If your throttle works, your brake cutoffs are fine. See E-Bike Motor Won’t Start After Braking? Check the Sensors.
- Pedal Assist Cutouts Under Load: If pedal assist works on flat ground but cuts out on hills, the issue is battery voltage sag or thermal throttling, not a missing sensor signal. See Why Your E-Bike Pedal Assist Cuts Out While Riding.
What To Do Right Now
- Check Assist Level: Ensure your handlebar display is set to Assist Level 1 or higher (Level 0 disables pedal power entirely).
- Inspect the Magnet Ring: Locate the plastic ring on the bottom bracket spindle near the left crank arm. Push it firmly toward the frame until it sits 1mm to 2mm away from the sensor pickup face.
- Verify Disc Rotation: Turn the pedals by hand and confirm that the magnet disc rotates in sync with the crank arm rather than slipping on the axle.
- Clean the Sensor Face: Wipe away caked grease, dirt, or metal shavings from the sensor pickup using isopropyl alcohol and a rag.
- Reseat the Connector: Trace the sensor wire to its 3-pin waterproof inline connector near the chainstay. Disconnect, inspect for bent pins or rust, and press firmly together aligning arrow markers.
When To Stop Immediately
- The motor unexpectedly engages full power when you are holding the bike stationary (“ghost pedaling”).
- You notice smoke, burning plastic odors, or hot wire insulation coming from the controller box.
- Wire conductors are exposed, frayed, or pinching against the chainring.
- Display shows active sensor short or controller hardware error codes.
What a Professional Will Check
- PAS Signal Voltage Test: Connect a multimeter to the sensor signal wire (usually blue or green) and ground pin. Slowly rotate the crank arms by hand to verify voltage toggles cleanly between 0V and 5V DC.
- 5V Power Supply Verification: Test the red wire pin on the PAS harness to confirm the controller is supplying a stable 5V DC reference voltage to the sensor.
- Display Settings Audit: Access the display advanced setup menu to confirm magnet count parameters match the installed disc hardware.
- Torque Sensor Millivolt Baseline: On torque-sensing setups, check the baseline output millivolts at rest versus under pedal pressure using a breakout harness.
Typical Repair Range
- Minor Repair (DIY Alignment / Cleaning / Settings Adjustment): $0 – $15 to realign the magnet disc, clean the pickup face, or replace a cracked split-ring magnet disc.
- Moderate Repair (PAS Sensor Replacement): $25 – $65 for a replacement cadence sensor assembly or sealed bottom bracket pickup.
- Major Repair (Integrated Torque Sensor / Controller Repair): $120 – $220+ if a shorted sensor circuit damaged the main controller board or the bike uses an integrated strain-gauge bottom bracket axle.
Related Symptom Escalators
- Broad pedal assist diagnostic and sensor troubleshooting: Troubleshooting E-Bike Pedal Assist (PAS) Failures and Delays
- Detailed cadence sensor diagnostic and replacement guide: E-Bike Cadence Sensor Failure: Pedal Assist Not Engaging
- Torque sensor vs. cadence sensor failure differences: E-Bike Torque Sensor Failure: Sluggish or No Assistance
- Troubleshooting when throttle fails alongside or independently: How to Fix an E-Bike Throttle That Isn’t Working and E-Bike Throttle Sensor Failure: Symptoms and Fixes
Ride Check
When your throttle drives the bike but pedal assist fails, you can rest easy knowing your battery, motor, and controller power stages are healthy. The issue is almost always a simple physical misalignment of the bottom bracket magnet disc or a loose low-voltage sensor connector. Pushing the magnet ring back into position and ensuring a 1mm to 2mm gap against the pickup face will restore seamless pedal-assisted riding in just a few minutes.