Torque sensor: weak, surging or no assist
SensorsTorque-sensor e-bikes
Torque-sensor systems (Bosch, Shimano, Yamaha, Brose, Bafang M-series, many bottom-bracket kits) measure how hard you push. When the zero point drifts or the sensor fails, you get weak assist, surging, or nothing. Most cases are a calibration issue caused by pressure on the pedals at power-on.
Warning: Do not attempt to open a drive unit to access the torque sensor. On bottom-bracket torque sensors, disconnect the battery before unplugging the sensor.
What you need
- Nothing but patience for the calibration steps
- Allen keys for crank arms if a bottom-bracket sensor is suspected
- Access to a dealer diagnostic tool for the final steps (Bosch, Shimano E-Tube, Yamaha, Bafang BESST)
LED patterns and what they mean
| Torque sensor initialisation not completed: pedal loaded at power-on | |
| Torque signal abnormal | |
| Torque sensor speed signal abnormal | |
| Can include torque sensor faults; dealer readout needed | |
| Zero point drifted upwards: sensor thinks you are already pushing | |
| Sensor signal noisy or crank/spider loose |
Step by step
1. Cold calibration: power on with no load
Switch off. Take both feet off the pedals, keep the bike upright and do not touch the cranks. Power on and wait 5-10 seconds before pedalling. Some systems (Bafang M, some Bosch) zero the sensor at every power-on; Shimano needs the cranks unloaded or it throws W013.
If OK: Assist feels normal again: the zero point had drifted because a foot was on the pedal at start-up. Make it a habit.
If not: Still weak, surging or absent. Continue.
Switch off. Take both feet off the pedals, keep the bike upright and do not touch the cranks. Power on and wait 5-10 seconds before pedalling. Some systems (Bafang M, some Bosch) zero the sensor at every power-on; Shimano needs the cranks unloaded or it throws W013.
If OK: Assist feels normal again: the zero point had drifted because a foot was on the pedal at start-up. Make it a habit.
If not: Still weak, surging or absent. Continue.
2. Assist level, ride mode and profile
Check the assist level is not the lowest, that Eco/Tour profiles have not been customised to a low support percentage (Bosch eBike Flow, Shimano E-Tube Ride, Bafang app), and that a 'child' or 'restricted' profile is not active.
If OK: Settings normal. Continue.
If not: Reset the profile to factory. Surprisingly common after an app update or a shop demo.
Check the assist level is not the lowest, that Eco/Tour profiles have not been customised to a low support percentage (Bosch eBike Flow, Shimano E-Tube Ride, Bafang app), and that a 'child' or 'restricted' profile is not active.
If OK: Settings normal. Continue.
If not: Reset the profile to factory. Surprisingly common after an app update or a shop demo.
3. Mechanical: cranks, spider and chainring
Grab each crank arm and rock it: no play. Check the chainring/spider bolts are tight. On bottom-bracket torque sensors, check the sensor's anti-rotation tab is seated and the lockring is tight.
If OK: No play, all tight. Continue.
If not: Loose crank or spider gives a surging signal. Tighten to spec (crank bolts typically 35-50 Nm). Retest.
Grab each crank arm and rock it: no play. Check the chainring/spider bolts are tight. On bottom-bracket torque sensors, check the sensor's anti-rotation tab is seated and the lockring is tight.
If OK: No play, all tight. Continue.
If not: Loose crank or spider gives a surging signal. Tighten to spec (crank bolts typically 35-50 Nm). Retest.
4. Battery under load
Ride with a full battery, then again at 30%. Weak assist that gets worse as the battery drops is a battery sag issue, not the torque sensor.
If OK: Assist equally weak at full and low charge. Continue.
If not: Assist fine at full, weak at low: battery ageing or a weak cell group.
Ride with a full battery, then again at 30%. Weak assist that gets worse as the battery drops is a battery sag issue, not the torque sensor.
If OK: Assist equally weak at full and low charge. Continue.
If not: Assist fine at full, weak at low: battery ageing or a weak cell group.
5. Error code correlation
Note any code: Shimano W013, Bafang 25/26, Bosch 5xx. Power-cycle with unloaded pedals and see if it returns.
If OK: No code, or a code that clears with correct start-up. Continue.
If not: Code returns every time: hardware fault in the sensor or its wiring. Bottom-bracket kits: step 6. Drive units: step 7.
Note any code: Shimano W013, Bafang 25/26, Bosch 5xx. Power-cycle with unloaded pedals and see if it returns.
If OK: No code, or a code that clears with correct start-up. Continue.
If not: Code returns every time: hardware fault in the sensor or its wiring. Bottom-bracket kits: step 6. Drive units: step 7.
6. Bottom-bracket torque sensor (kits, some hub-motor bikes)
Battery off. Unplug the sensor (usually 5-6 pins). Check for corrosion. Reconnect, battery on, and if the display supports it, watch the torque value (some KT/Lishui displays show a raw torque number). Push the pedal: the number should rise smoothly and fall back to the same baseline.
If OK: Value rises and returns to baseline: sensor good. The controller's torque scaling may be set wrong; check the display torque parameters.
If not: No change, or baseline drifts and does not return: sensor faulty. Replace the bottom-bracket sensor unit (50-150 EUR/USD).
Battery off. Unplug the sensor (usually 5-6 pins). Check for corrosion. Reconnect, battery on, and if the display supports it, watch the torque value (some KT/Lishui displays show a raw torque number). Push the pedal: the number should rise smoothly and fall back to the same baseline.
If OK: Value rises and returns to baseline: sensor good. The controller's torque scaling may be set wrong; check the display torque parameters.
If not: No change, or baseline drifts and does not return: sensor faulty. Replace the bottom-bracket sensor unit (50-150 EUR/USD).
7. Dealer calibration and diagnostic readout
A dealer tool can read the live torque value, run a torque sensor calibration/zeroing routine (Bosch, Yamaha and Bafang have one; Shimano re-initialises on start-up), and read stored errors.
If OK: Calibration restores full assist: done. Ask them to note the pre- and post-values.
If not: Calibration fails or the live value is stuck/noisy: torque sensor hardware inside the drive unit. On most units it is not separately replaceable; the options are a specialist rebuilder or a new drive unit.
A dealer tool can read the live torque value, run a torque sensor calibration/zeroing routine (Bosch, Yamaha and Bafang have one; Shimano re-initialises on start-up), and read stored errors.
If OK: Calibration restores full assist: done. Ask them to note the pre- and post-values.
If not: Calibration fails or the live value is stuck/noisy: torque sensor hardware inside the drive unit. On most units it is not separately replaceable; the options are a specialist rebuilder or a new drive unit.
8. Ride test
Ride 5 km, alternate between light spinning and hard pushes on a hill.
If OK: Assist proportional to effort, no surging: done.
If not: Surging on hard pushes only: chain/gear skipping can feel like sensor surging; check the drivetrain.
Ride 5 km, alternate between light spinning and hard pushes on a hill.
If OK: Assist proportional to effort, no surging: done.
If not: Surging on hard pushes only: chain/gear skipping can feel like sensor surging; check the drivetrain.
What you now know for sure
- Zero-point drift (calibration): Pedal loaded at power-on; assist weak or W013. → Power on with feet off the pedals. Free.
- Settings / profile: Support percentage or profile lowered in the app. → Reset to factory profile.
- Mechanical play: Loose crank, spider or sensor lockring; surging. → Tighten to spec.
- Bottom-bracket sensor (kits): No or drifting raw signal. → Replace sensor unit. 50-150.
- Drive-unit torque sensor: Dealer calibration fails; live value stuck. → Rebuilder or new drive unit.
Frequently asked
Why should I not touch the pedals when switching the bike on?
The system records the sensor's reading at power-on as 'zero effort'. If your foot is on a pedal, that load becomes the baseline and every ride afterwards feels weak because the sensor subtracts it. Shimano flags it as W013.
Can a torque sensor be recalibrated at home?
The zeroing at power-on is the only home calibration. Full scaling calibration needs the dealer tool. Bottom-bracket kit sensors usually have no calibration at all; they either work or get replaced.
Is surging assist always the torque sensor?
No. A loose crank, a skipping chain, a worn freewheel and a battery on the edge of its low-voltage cut can all feel like surging. Check the mechanical side and the battery before condemning the sensor.
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.