Brake cut-off sensor stuck: no assist, brake icon on the display
Brakes & cut-offAny e-bike with brake sensors
Every e-bike with brake sensors kills the motor when a lever is pulled. A sensor that thinks the lever is always pulled gives you a bike that powers on but refuses to assist. The unplug test settles it in two minutes.
Warning: Brake sensors are a safety device. Riding with them bypassed is a temporary test only, never a fix: a stuck throttle or a cadence sensor that keeps the motor running while you brake is how people crash. Refit or replace the sensor before normal riding.
What you need
- Nothing for the unplug test
- Multimeter (optional, for the sensor signal)
- Small magnet or a spare brake sensor
- Cable ties
LED patterns and what they mean
| Brake sensor active / brake detected at power-on | |
| Throttle not at rest; looks identical to a rider (no assist), so test both | |
| Brake sensor fault or brake held at power-on | |
| Brake input active | |
| No brake sensors on most mid-drives; a brake symbol means a brake-light or ABS function | |
| Generic brake-cut indicator |
Step by step
1. Look for the brake symbol
Power on, hands off the levers, and look at the display for a brake icon, a hand, an 'E', or codes 03/E07.
If OK: No brake indication. The brake sensors are probably not the cause, but do the unplug test anyway as some displays have no icon. Continue.
If not: Brake icon shown with the levers released: a sensor is reporting 'pulled'. Continue with step 2.
Power on, hands off the levers, and look at the display for a brake icon, a hand, an 'E', or codes 03/E07.
If OK: No brake indication. The brake sensors are probably not the cause, but do the unplug test anyway as some displays have no icon. Continue.
If not: Brake icon shown with the levers released: a sensor is reporting 'pulled'. Continue with step 2.
2. The unplug test
Find the brake sensor plugs: usually two 2-pin or 3-pin connectors near the stem or on the 1-to-4 harness. Unplug one. Try the throttle or pedal. Plug it back, unplug the other, try again.
If OK: Motor runs with a specific sensor unplugged: that sensor, lever or its wire is the fault. Go to step 3.
If not: No change with both unplugged: brake sensors are cleared. Plug both back and return to the hub-motor or display-error-no-assist flow.
Find the brake sensor plugs: usually two 2-pin or 3-pin connectors near the stem or on the 1-to-4 harness. Unplug one. Try the throttle or pedal. Plug it back, unplug the other, try again.
If OK: Motor runs with a specific sensor unplugged: that sensor, lever or its wire is the fault. Go to step 3.
If not: No change with both unplugged: brake sensors are cleared. Plug both back and return to the hub-motor or display-error-no-assist flow.
3. Mechanical vs hydraulic sensor type
Mechanical lever with a sensor built in: a magnet in the lever blade passes a reed/hall switch in the lever body. Hydraulic lever: usually a separate small sensor block with a magnet glued to the lever, or a built-in switch (Tektro, Shimano e-bike levers) that closes when the lever moves.
If OK: Type identified. Continue.
If not: Cannot find a sensor on hydraulic levers: it may be a hidden switch in the lever body with a short cable exiting near the clamp.
Mechanical lever with a sensor built in: a magnet in the lever blade passes a reed/hall switch in the lever body. Hydraulic lever: usually a separate small sensor block with a magnet glued to the lever, or a built-in switch (Tektro, Shimano e-bike levers) that closes when the lever moves.
If OK: Type identified. Continue.
If not: Cannot find a sensor on hydraulic levers: it may be a hidden switch in the lever body with a short cable exiting near the clamp.
4. Magnet position and lever return
Check the lever returns fully to rest (sticky pivot, over-tight reach adjuster, or a bent blade). Check the glue-on magnet is still there and sits over the sensor at rest, then moves away when the lever is pulled (or vice versa depending on type: normally-open vs normally-closed).
If OK: Lever returns, magnet in the right place. Continue.
If not: Lever not returning: lubricate pivot, back off reach adjuster, bleed if the piston is sticky. Magnet lost or shifted: re-glue it in the rest position and test.
Check the lever returns fully to rest (sticky pivot, over-tight reach adjuster, or a bent blade). Check the glue-on magnet is still there and sits over the sensor at rest, then moves away when the lever is pulled (or vice versa depending on type: normally-open vs normally-closed).
If OK: Lever returns, magnet in the right place. Continue.
If not: Lever not returning: lubricate pivot, back off reach adjuster, bleed if the piston is sticky. Magnet lost or shifted: re-glue it in the rest position and test.
5. Signal test with a meter
With the sensor plugged in and the display on, measure the sensor signal to ground. Normally-open type: near 5V at rest, drops to 0V when pulled. Normally-closed: 0V at rest, 5V when pulled. Compare both levers: they should behave the same.
If OK: Signal changes correctly with the lever: sensor good. If the icon still shows, the controller input may be at fault (step 7).
If not: Signal stuck: sensor dead or wire shorted/broken. Replace the sensor (5-20 EUR/USD) or the whole lever if it is built in (20-60).
With the sensor plugged in and the display on, measure the sensor signal to ground. Normally-open type: near 5V at rest, drops to 0V when pulled. Normally-closed: 0V at rest, 5V when pulled. Compare both levers: they should behave the same.
If OK: Signal changes correctly with the lever: sensor good. If the icon still shows, the controller input may be at fault (step 7).
If not: Signal stuck: sensor dead or wire shorted/broken. Replace the sensor (5-20 EUR/USD) or the whole lever if it is built in (20-60).
6. Wiring: chafe, crush and water
Follow both brake wires to the harness. Look for the cable pinched under the handlebar clamp, chafed by the fork on full lock, or water in the connector.
If OK: Wires intact and dry. Continue.
If not: Repair or replace; a pinched brake wire that shorts intermittently gives random assist cut-outs on bumps.
Follow both brake wires to the harness. Look for the cable pinched under the handlebar clamp, chafed by the fork on full lock, or water in the connector.
If OK: Wires intact and dry. Continue.
If not: Repair or replace; a pinched brake wire that shorts intermittently gives random assist cut-outs on bumps.
7. Polarity and controller input
Some controllers accept normally-open only, others need a setting (KT: 'brake polarity' parameter). If you replaced a sensor with the wrong type, the display will show the brake as pulled at rest. If a known-good sensor of the correct type still shows pulled, the controller input is at fault.
If OK: Correct type and setting: done.
If not: Wrong type: swap the sensor for the right type or change the polarity parameter. Controller input dead: the controller can usually be run with that input empty, but a faulty input that toggles randomly means replacing the controller.
Some controllers accept normally-open only, others need a setting (KT: 'brake polarity' parameter). If you replaced a sensor with the wrong type, the display will show the brake as pulled at rest. If a known-good sensor of the correct type still shows pulled, the controller input is at fault.
If OK: Correct type and setting: done.
If not: Wrong type: swap the sensor for the right type or change the polarity parameter. Controller input dead: the controller can usually be run with that input empty, but a faulty input that toggles randomly means replacing the controller.
8. Temporary bypass, only for a short test ride
If you must ride home: leave the faulty sensor unplugged; the other lever still cuts the motor. Ride slowly, low assist, no throttle. Never seal the plug or tape a magnet on permanently.
If OK: You get home. Order the part and refit before the next ride.
If not: Both sensors faulty: do not ride with power; walk it or pedal unassisted.
If you must ride home: leave the faulty sensor unplugged; the other lever still cuts the motor. Ride slowly, low assist, no throttle. Never seal the plug or tape a magnet on permanently.
If OK: You get home. Order the part and refit before the next ride.
If not: Both sensors faulty: do not ride with power; walk it or pedal unassisted.
What you now know for sure
- Lever not returning / magnet moved: Sensor is fine, the lever keeps triggering it. → Free pivot, reposition magnet.
- Brake sensor: Signal stuck, unplugging it restores assist. → Replace sensor or lever. 5-60.
- Brake wire: Pinched, chafed or wet connector. → Repair/replace; reroute the cable.
- Wrong sensor type / polarity setting: Replacement sensor is NC where NO is needed (or the reverse). → Swap type or change the controller/display parameter.
- Controller brake input: Known-good sensor still reads pulled. → Replace controller.
Manuals and video
Videos found on YouTube and checked automatically; they are other people's work, not ours.
- Fix Your EBike/Bike brake under 15 seconds (How to, tutorial) #ebike #tips #bike #brake TN Ebike Canada
- How to Disconnect Brake Sensors on Ridstar Q20 #ebike #wheelies ridetards
- How to Install Brake Sensor #ebike #bicycle #cycling #installation to7motor
Frequently asked
Is it safe to ride with the brake sensors unplugged?
Only as a short, slow test. The brake cut-off is the only thing that stops the motor if a throttle sticks or a cadence sensor keeps the motor running as you brake. Refit or replace the sensor before normal use.
Why does the brake icon appear only when I hit a bump?
An intermittent wire or a loose magnet. Check the cable where it passes the handlebar clamp and the glue on the magnet.
Which Bafang error code is the brake?
03 on most Bafang displays. Some C-series displays show 04 for a throttle that is not at rest at power-on; to a rider both look like 'no assist', so unplug the throttle too if 04 is shown.
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.