Speed sensor: no assist, cut-out at speed, or speed stuck at 0
SensorsAny e-bike
The speed sensor tells the controller how fast the wheel turns. When it fails, the system either refuses to assist, assists briefly then stops, or shows 0 km/h. Bosch 503/504, Bafang 21 and Shimano W011 all point here. Nearly every case is a magnet position, a gap, or a chafed cable.
Warning: Rim- and disc-magnet systems sit next to the brake rotor. Do not touch a hot rotor, and re-check that a repositioned magnet cannot touch the rotor or spokes.
What you need
- Ruler or feeler gauge (mm)
- Small Allen/Torx keys for the sensor
- Multimeter (optional, for the sensor signal on hub systems)
- Zip ties
LED patterns and what they mean
| Speed sensor signal error: sensor or magnet missing/misaligned | |
| Speed signal implausible: magnet too far, tuning dongle detected, or wheel size wrong | |
| Speed sensor abnormal: no pulses while motor is running | |
| Speed sensor signal cannot be detected | |
| Speed sensor / magnet fault | |
| Speed pickup missing but the controller does not require it (hub systems with internal hall speed) |
Step by step
1. Identify your sensor type
Look at the rear chainstay. Spoke magnet + small sensor = classic external sensor. Nothing on the spokes but a sensor at the disc rotor = rim/disc-magnet sensor (Bosch rim magnet on the rotor, Shimano, newer Bafang). No external sensor at all on a hub motor = speed comes from the motor's hall sensors.
If OK: Sensor type identified. Continue.
If not: Hub motor with no external sensor and speed showing 0: the speed pulse comes from the motor halls or an internal speed sensor on the hall wire (white wire). Go to the hub-motor flow, hall test.
Look at the rear chainstay. Spoke magnet + small sensor = classic external sensor. Nothing on the spokes but a sensor at the disc rotor = rim/disc-magnet sensor (Bosch rim magnet on the rotor, Shimano, newer Bafang). No external sensor at all on a hub motor = speed comes from the motor's hall sensors.
If OK: Sensor type identified. Continue.
If not: Hub motor with no external sensor and speed showing 0: the speed pulse comes from the motor halls or an internal speed sensor on the hall wire (white wire). Go to the hub-motor flow, hall test.
2. Is the magnet present, tight and in line?
Spoke magnet: it should be clamped on the spoke, the magnet face pointing at the sensor, and pass directly across the sensor's marked face. Rim/disc magnet: check the magnet ring on the rotor is complete, not chipped, and the rotor is the correct one for the sensor.
If OK: Magnet present and aligned. Continue.
If not: Magnet missing (they fall off), rotated, or slid down the spoke: refit it. Rotor changed to a non-magnet rotor: fit the correct rotor or switch to a spoke magnet system.
Spoke magnet: it should be clamped on the spoke, the magnet face pointing at the sensor, and pass directly across the sensor's marked face. Rim/disc magnet: check the magnet ring on the rotor is complete, not chipped, and the rotor is the correct one for the sensor.
If OK: Magnet present and aligned. Continue.
If not: Magnet missing (they fall off), rotated, or slid down the spoke: refit it. Rotor changed to a non-magnet rotor: fit the correct rotor or switch to a spoke magnet system.
3. Gap between magnet and sensor
Measure the gap at the closest point. Bosch spoke magnet: 5-17 mm; Bosch rim magnet: a few mm with the rotor passing the sensor face. Bafang/hub kits: 5-10 mm. Shimano: as per the sticker on the sensor, typically under 10 mm. Also check the magnet does not sit above or below the sensor's active area.
If OK: Gap in spec. Continue.
If not: Gap too big or magnet off-centre: bend the spoke magnet slightly toward the sensor, or move the sensor on its slot. Re-test by spinning the wheel and watching the speed readout.
Measure the gap at the closest point. Bosch spoke magnet: 5-17 mm; Bosch rim magnet: a few mm with the rotor passing the sensor face. Bafang/hub kits: 5-10 mm. Shimano: as per the sticker on the sensor, typically under 10 mm. Also check the magnet does not sit above or below the sensor's active area.
If OK: Gap in spec. Continue.
If not: Gap too big or magnet off-centre: bend the spoke magnet slightly toward the sensor, or move the sensor on its slot. Re-test by spinning the wheel and watching the speed readout.
4. Spin test: does speed appear?
Rear wheel in the air, system on, spin the wheel briskly and watch the display.
If OK: Speed appears and rises with wheel speed. Sensor is working. If assist still cuts out, go to step 7.
If not: Speed stays 0: continue to the cable and sensor tests.
Rear wheel in the air, system on, spin the wheel briskly and watch the display.
If OK: Speed appears and rises with wheel speed. Sensor is working. If assist still cuts out, go to step 7.
If not: Speed stays 0: continue to the cable and sensor tests.
5. Cable and connector
Trace the sensor cable from the sensor to the motor or controller. Look for chafe from the crank or tyre, a cut from a wheel change, or a connector pulled loose. On mid-drives the sensor plugs into the drive unit behind the cover. On hub kits it is a small 3-pin plug on the harness.
If OK: Cable intact, connector seated. Continue.
If not: Damaged or unplugged: repair or replace. A chafed sensor cable near the crank is the number-one cause of Bafang 21.
Trace the sensor cable from the sensor to the motor or controller. Look for chafe from the crank or tyre, a cut from a wheel change, or a connector pulled loose. On mid-drives the sensor plugs into the drive unit behind the cover. On hub kits it is a small 3-pin plug on the harness.
If OK: Cable intact, connector seated. Continue.
If not: Damaged or unplugged: repair or replace. A chafed sensor cable near the crank is the number-one cause of Bafang 21.
6. Sensor test
Hub/generic 3-wire sensors: with power on, measure red-black (5V or 12V) and signal-black: the signal should switch between high and low each time the magnet passes. No meter: swap in a known-good sensor (10-20 EUR/USD). Bosch/Shimano sensors: swap is the practical test; dealers can also read the sensor status in the diagnostic tool.
If OK: Signal toggles: sensor good. If speed still shows 0 the controller's speed input is faulty; on hub kits that is a controller replacement.
If not: No supply voltage: controller/drive unit is not powering the sensor, check the connector pins then suspect the controller. Supply present but no toggle: sensor dead, replace it.
Hub/generic 3-wire sensors: with power on, measure red-black (5V or 12V) and signal-black: the signal should switch between high and low each time the magnet passes. No meter: swap in a known-good sensor (10-20 EUR/USD). Bosch/Shimano sensors: swap is the practical test; dealers can also read the sensor status in the diagnostic tool.
If OK: Signal toggles: sensor good. If speed still shows 0 the controller's speed input is faulty; on hub kits that is a controller replacement.
If not: No supply voltage: controller/drive unit is not powering the sensor, check the connector pins then suspect the controller. Supply present but no toggle: sensor dead, replace it.
7. Assist cuts at a certain speed, or 504 on Bosch
Check the wheel circumference setting in the display (wrong size = wrong speed = early cut-off). Check for a tuning device (dongle, magnet relocated to the crank): Bosch 504 flags manipulation and locks assist. Remove any dongle and ride 90 minutes without assist to clear the lock on newer Bosch systems, or have the dealer clear it.
If OK: Wheel size correct, no dongle, assist runs to the legal limit: done.
If not: Wrong wheel size: correct it. Dongle present: remove it; repeated 504s need the dealer to reset the drive unit.
Check the wheel circumference setting in the display (wrong size = wrong speed = early cut-off). Check for a tuning device (dongle, magnet relocated to the crank): Bosch 504 flags manipulation and locks assist. Remove any dongle and ride 90 minutes without assist to clear the lock on newer Bosch systems, or have the dealer clear it.
If OK: Wheel size correct, no dongle, assist runs to the legal limit: done.
If not: Wrong wheel size: correct it. Dongle present: remove it; repeated 504s need the dealer to reset the drive unit.
8. Ride test
Ride 5 km including a fast downhill and a stop-start section. Watch the speed readout and assist.
If OK: Consistent speed and assist: done.
If not: Speed drops to 0 intermittently: magnet gap opens under load (loose sensor, bent spoke magnet) or an intermittent cable. Zip-tie the cable and tighten the magnet.
Ride 5 km including a fast downhill and a stop-start section. Watch the speed readout and assist.
If OK: Consistent speed and assist: done.
If not: Speed drops to 0 intermittently: magnet gap opens under load (loose sensor, bent spoke magnet) or an intermittent cable. Zip-tie the cable and tighten the magnet.
What you now know for sure
- Magnet position or gap: Magnet loose, rotated, missing or too far. Most common by far. → Refit and set the gap. Free.
- Sensor cable: Chafed, cut or unplugged. → Repair or replace the sensor assembly. 15-40.
- Sensor itself: Supply present, no signal toggle. → Replace sensor. 10-60 depending on brand.
- Wrong setting or tuning device: Wrong wheel size, or Bosch 504 manipulation lock. → Correct wheel size, remove dongle, dealer reset if locked.
- Controller / drive unit input: Good sensor and cable but no speed registers. → Hub kit: replace controller. Mid-drive: dealer diagnostics.
Manuals and video
Videos found on YouTube and checked automatically; they are other people's work, not ours.
- eBike Speed Sensor, Speed Magnet Troubleshooting: Fly Rides' Two Minute Tips! Fly Rides USA | Electric Bikes
- How to fix Error 503 on Bosch #ebike #bicyclemaintenance #shorts Speed sensor alignment Cargo Bike Repairs
- check speed sensor to press it， B20/A20 Electric Bike #rowerelektryczny #ebike #automobile Yan Kevin
Frequently asked
Why does my e-bike assist for a few seconds and then stop?
The controller starts assisting on pedal input, then expects speed pulses to confirm the wheel is turning. With no speed signal it assumes a fault and shuts off after a few seconds, often with Bafang 21 or Bosch 503. Fix the magnet or sensor.
Bosch 504 keeps coming back after I removed the tuning dongle. What now?
Newer Bosch systems store the manipulation event and limit assist until a penalty period has passed (ride about 90 minutes without assist) or a dealer clears it. Also check the magnet gap; a magnet that is too far away can trigger 504 without any dongle.
Does a hub motor need a speed sensor?
Many hub systems take speed from the motor's hall sensors and have no external sensor. Others use a fourth 'speed' wire inside the hall cable. If yours shows 0 speed but assists normally, it is a display setting or hall speed wire issue, not a safety cut-out.
Still stuck? Take a photo of the display and show it to a local e-bike workshop. Bring the bike's system name (e.g. Bosch Performance Line CX) and the exact code; that saves diagnostic time and money.