TL;DR
- Most pedal assist failures come from a wrong assist setting, a loose connector, a dirty or misaligned magnet ring, or a stuck brake cutoff sensor, and you can check all four at home.
- If the throttle works but pedal assist does not, the fault is likely in the pedal sensor system, not the motor or controller.
- Rad Power Bikes lists a gap of about 3 mm between the magnet ring and the sensor (Rad Power Bikes Support, 2026).
- The full process is 7 steps and takes about 30 minutes with basic tools.
- If every step fails, the PAS sensor itself is the likely culprit, and replacing it is usually a small repair.
What You Need Before You Start
You need very little to run these checks. Gather these items first:
- A small Allen key set, since most sensor brackets use hex bolts.
- A clean rag and a soft brush for removing grime.
- A fully charged battery, so low voltage does not confuse the diagnosis.
- Your phone, to film the crank area before you touch anything. Rad Power Bikes suggests recording your steps in case support needs to review them (Rad Power Bikes Support, 2026).
Pedal assist system (PAS) is the part of an e-bike that detects your pedaling and tells the controller when to run the motor. A cadence sensor (also called a PAS sensor) does this by reading magnets on a ring that spins with the crank.
Step 1: Check the Display and the Assist Level
Turn the bike on and confirm the assist level is set to 1 or higher. Level 0 means no motor help, and it is the most common reason pedal assist seems dead. Leoguar Ebikes calls this the most common culprit.
Next, look at the display for error codes. A code points you to the faulty part faster than guessing, and the bike manual lists what each code means.
One more setting matters on kit bikes. A forum user on Endless Sphere reported spending six hours on a dead PAS before remembering the magnet setting in the display menu. It’s worth checking that value if your display has one (Endless Sphere, 2026).
Step 2: Reseat the Battery and Reconnect the Sensor Cable
A loose battery or connector can cut power to the whole system, so reseat both before anything else. Remove the battery, check the contacts for dirt, and click it back in firmly.
A slightly loose battery can cause intermittent power cuts to the entire system, including the pedal sensor.
Then follow the sensor cable from the crank area to where it plugs in, and press each connector until it seats. Look for cuts, kinks, or pinched wire near the frame. Damaged wiring is one of the usual causes of PAS faults (Upway, 2026).
Step 3: Run the Throttle Test to Narrow Down the Fault
The throttle test tells you which half of the system is broken. If your bike has a throttle, twist or press it with the wheel off the ground. If the motor spins, the motor, battery, and controller are working.
That result points to the PAS input path: the sensor, the magnet ring, or the wiring between them.
If the throttle also does nothing, the problem is more likely the battery, controller, or a brake cutoff. Skip ahead to Step 6.
Safety note: if the motor engages on its own or unpredictably at any point, stop riding. EM3EV advises against riding an e-bike with unpredictable motor engagement (EM3EV, 2026).
Step 4: Inspect and Clean the Magnet Ring
A dirty, cracked, or incomplete magnet ring stops the sensor from seeing your pedaling. The magnet ring (also called the magnet disc) is a plastic disc with small magnets, mounted on the crank spindle next to the PAS sensor.
On many bikes, you will find both parts behind the right crank arm. Wipe the ring and the sensor with a dry rag.
Dirt, mud, and debris can interfere with cadence sensors and magnet rings, and a dirty crank area gives the controller a poor signal.
Spin the cranks slowly and look at each magnet. Missing magnets, cracks, or a wobbling ring mean the ring needs replacing (Upway, 2026).
Step 5: Set the Gap Between the Magnet Ring and the Sensor
The gap between the magnet ring and the sensor should be small and even, about 3 mm on Rad Power Bikes models (Rad Power Bikes Support, 2026). Other brands quote 1 to 3 mm (Leoguar Ebikes, 2026).
Rad Power Bikes compares 3 mm to two stacked credit cards. Use that as a quick field gauge.
To fix a bad gap:
- Loosen the sensor bracket bolt slightly.
- Move the sensor until the gap is even all the way around the ring.
- Tighten the bolt, then spin the cranks to confirm nothing rubs.
If the ring sits too far from the sensor, gently push it closer while keeping it square to the bottom bracket spindle, so it does not wobble when spun.
Test ride the bike. A misaligned sensor often makes the motor cut in and out, or fail to engage at all (Upway, 2026).
Step 6: Test the Brake Cutoff Sensors
Brake cutoff sensors stop the motor whenever a brake lever is pulled, and a stuck one blocks pedal assist completely. Brake cut-off sensors that are dirty or stuck often prevent pedal assist from working.
Check each brake lever:
- The lever should return fully to its rest position.
- The sensor wire should be plugged in and undamaged.
- The sensor area should be clean and free of grease.
Unplug the brake sensor connectors one at a time and test the bike with the wheel off the ground. If assist returns, the sensor you just unplugged is the cause. Replace it before riding, since the cutoff is a safety feature.
Step 7: Replace the PAS Sensor If Everything Else Checks Out
If the ring is clean, the gap is right, the wiring is intact, and the brakes are clear, the sensor has likely failed. Upway lists “faulty sensor” as the cause when all visible parts look normal but assist still does not work (Upway, 2026).
Cadence sensors are inexpensive and many owners can swap them in about 20 minutes. Match the connector type and the sensor model to your bike before ordering.
Bikes with an integrated torque sensor or a motor-internal PAS are different. Integrated torque sensors and motor-integrated PAS systems may need a different service process and should not be treated as generic plug-in parts. Take those bikes to a dealer.
Common Pedal Assist Problems and Their Fixes
| Symptom | Likely Cause | Fix |
|---|---|---|
| No assist, throttle works | Magnet ring gap or dirty sensor | Clean and realign (Steps 4-5) |
| Assist cuts in and out | Misaligned sensor or loose connector | Reseat cable, adjust gap |
| No assist, throttle also dead | Brake cutoff or battery contact | Check brake sensors, reseat battery |
| Assist only works with the throttle | Failed PAS sensor or broken wire | Test wiring, replace sensor |
| Motor runs on randomly | Controller or sensor fault | Stop riding, see a technician |
Frequently Asked Questions About Ebike Pedal Assist Problems
A working throttle with dead pedal assist usually points to the PAS input path: the sensor, the magnet ring, or the wiring. EM3EV notes this narrows the problem but does not prove the controller is healthy (EM3EV, 2026). Start with a clean and a gap check.
A PAS sensor is likely bad when the display, battery, wiring, brake cutoffs, and magnet ring all check out but assist still fails. Upway gives this same diagnosis (Upway, 2026). A technician can confirm it with a multimeter.
A misaligned sensor, a loose connector, or a damaged magnet ring most often causes intermittent assist. Reseat the connectors and check the gap first. Also confirm the battery sits firmly, since a loose battery can cause power drops (Leoguar Ebikes, 2026).
Yes. Dirt and debris around the crank area weaken the signal the controller receives (Upway, 2026). Wiping the sensor and ring with a dry rag takes two minutes and fixes many cases.
On most cadence-sensor bikes, yes. You unplug the old sensor, unbolt it, fit the new one, and set the gap. Torque sensors and sensors built into the motor should be serviced by a dealer, since they are not generic plug-in parts (EM3EV, 2026).
You can ride the bike as a regular bicycle if the motor stays off, though the extra weight makes hills harder. Do not ride if the motor engages unpredictably.
Quick Recap of the Fix Order
- Check the display and set the assist level above 0.
- Reseat the battery and the sensor cable.
- Run the throttle test to narrow down the fault.
- Clean the magnet ring and sensor.
- Set the gap to roughly 1-3 mm (3 mm on Rad Power Bikes).
- Test the brake cutoff sensors.
- Replace the PAS sensor if all else fails.
References
- Rad Power Bikes Support, “Pedal assist system (PAS) troubleshooting guide”: https://support.radpowerbikes.com/Maintenance_and_troubleshooting/Throttle_and_pedal_assist/Pedal_assist_system_(PAS)_troubleshooting_guide
- Upway, “How To Fix A Pedal Assist Sensor On An E-Bike”: https://upway.co/blogs/news/how-to-fix-a-pedal-assist-sensor-on-an-e-bike
- EM3EV, “eBike PAS Sensor Not Working? Causes & Safe Fixes”: https://em3ev.com/?p=43259
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