Most electric cargo-bike faults can be sorted safely by following one sequence: switch the system off, secure the bike and cargo, check battery seating and clean, dry connectors, record the exact error code and system generation, then test again at low speed. Do not open a Bosch battery or motor. Persistent electrical faults, damaged wiring, hydraulic leaks, or any uncertain braking require an authorized dealer or qualified bicycle service shop.
Start with a safe, repeatable diagnosis
A cargo bike carries more weight than a conventional bicycle, so a small electrical or brake problem can become a serious control problem. Before touching a connector, wheel, rotor, caliper, or motor:
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- Switch the drive system off.
- Secure the cargo so it cannot shift while the bike is being inspected.
- Remove the battery only when the manufacturer’s procedure calls for it.
- Keep fingers, clothing, and tools away from moving wheels.
- Assume the rotor and caliper may be hot; rotor edges are sharp.
- Never bypass a brake cut-off or ride when braking performance is uncertain.
Write down the bike model, battery and drive-unit version, display model, and the exact wording or number of any error. A code from one manufacturer is not interchangeable with the same number on another system.
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A quick triage for a bike that will not work
- Power down completely. Turn the system off and wait for it to shut down rather than repeatedly pressing the control button.
- Check the battery. Confirm that it is inserted fully, locked as designed, and showing charge on its indicator.
- Inspect contacts. Look for dust, moisture, corrosion, bent parts, heat discoloration, or physical damage. Contacts and plugs must be clean and dry.
- Check the display and control. Make sure the display is seated on its mount and that external cables are aligned and fully inserted.
- Photograph an error. Capture the code before cycling power, then use the manual or app for that exact bike-system generation.
- Retest without riding. If the system starts, check controls while the bike is stationary. A fault that returns, a hot connector, or water intrusion is a service issue.
When the battery will not charge or the bike will not turn on
Confirm charge and battery seating
Tern’s Bosch troubleshooting guide identifies a depleted battery, a loose connection, and a display issue as possible causes of a no-start condition. Push the battery fully into its mount and engage the lock. Do not force a pack that does not align; stop and inspect the mount for an obstruction or damage.
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Check the battery’s own indicators before blaming the motor. A battery that appears to be installed can still have insufficient charge for the system to start. If the pack was recently exposed to rain, condensation, road spray, or a wash, allow the contacts to dry naturally before reconnecting. Do not scrape contacts with a metal tool or spray lubricant into the connector.
Use the indicator pattern instead of guessing
| Battery indication | Meaning documented by Tern | What to do |
|---|---|---|
| LEDs 1, 3, and 5 flashing | The pack is outside the 0–40°C (32–104°F) charging range. | Move the battery to a place within that range and let it reach the temperature naturally before charging. |
| No LEDs when switching on | About 5% charge or less. | Charge the battery with the specified charger and check that the charger and connection are seated correctly. |
| LEDs 2 and 4 flashing | A battery defect has been detected. | Stop troubleshooting by repeatedly reconnecting it and contact an authorized Tern dealer or Bosch service center. |
These patterns are from Tern’s current support guidance for Bosch systems on Tern bikes. Other brands and generations can use different indicators, so do not apply this table to another battery without confirming its manual.
Inspect the display and its cable
A dark display does not prove that the battery is dead. Check that the display is seated on its mount, that the mount and contacts are clean and dry, and that any display battery is serviceable. Follow the cable from the display or controller and look for a plug that has pulled out, a crushed section, or a connector that was fitted at an angle during transport.
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Do not twist a plug to make it fit. Misalignment can damage small contacts and turn a simple loose connection into a harness repair. If the display has its own battery, replace or charge that battery only as specified by the display manual.
Perform a complete shutdown
When the battery and connections look normal, Tern recommends a complete shutdown, removal of the battery for a few minutes, and correct reinstallation. This can clear a temporary communication state, but it cannot repair a damaged pack, motor, controller, or cable. If the display remains dark or the same error returns, arrange service rather than repeating the reset indefinitely.
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Tern’s explicit safety instruction is: “Do not open the Bosch motor or battery.” A lithium-ion battery can contain high-energy cells and protective electronics; the motor and controller also require system-specific tools and procedures. Opening either component can create shock, fire, water-ingress, and warranty risks.
When the motor cuts out or shows an electrical error
Understand Error 21 without overgeneralizing it
Rad Power Bikes defines Error 21 as an abnormal motor-current condition or a problem in the power path from the battery to the controller. The code can appear when the bike is pushed harder than recommended, such as on a steep hill or with excessive cargo load. That definition applies to the system documented by Rad Power Bikes; it does not establish what Error 21 means on a Bosch, Shimano, or another drive system.
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Inspect the power path safely
- Switch the bike off.
- Remove the battery according to the manufacturer’s instructions.
- Press the MODE or power button to discharge residual power, following the procedure specified for the bike.
- Inspect the battery terminals, battery connector, and motor connector for dirt, moisture, corrosion, bent contacts, heat damage, or a loose fit.
- Reconnect each plug straight, without twisting or forcing it. The connector should be clean and dry before it is joined.
- Reinstall and lock the battery, then power up while the bike is stationary.
If the code returns immediately, the motor cuts out repeatedly, a connector becomes hot, or there is evidence of water intrusion, stop. Do not bridge terminals, substitute a random charger, or continue riding to see whether the fault clears. A qualified technician needs to test the battery, controller, harness, and motor as a matched system.
Separate overload symptoms from a failed component
A cutout under a steep hill or unusually heavy load can indicate that the system detected an abnormal current condition, but the same symptom can also result from a damaged connector or internal fault. Reduce the load only as a diagnostic precaution; do not treat a cutout as proof that the bike is safe for normal use. If normal, moderate assistance still causes the code, service is the correct next step.
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How to fix a blank or failing display
If the display is completely dark
- Verify battery charge and seating first.
- Check the display’s mount and contacts for dirt or moisture.
- Confirm that a display battery, if fitted, is charged or serviceable.
- Inspect external display and controller cables for looseness, damage, or incorrect alignment.
- Perform the complete shutdown and battery-removal sequence described above.
A display that went blank after a rack or cargo-loading operation may have a cable that was pulled or pinched. Reconnect only plugs designed to be user-accessible; do not open sealed housings or splice a harness.
If the display powers up with an error
Photograph the code before switching the system off. Then look it up in the exact Bosch, Shimano, or bicycle manual for the bike’s generation. Tern publishes separate GSD manuals for Bosch batteries, chargers, drive units, and displays, which is why a model-generation check matters before ordering a part or following a reset procedure.
Do not erase the code repeatedly before recording it. The history can help a dealer distinguish a low-voltage event, a communication interruption, a sensor issue, and a component fault.
Brake problems: rubbing, squealing, and weak stopping
Make a stationary brake check
With the bike supported and the system off, squeeze each brake lever and observe the wheel, rotor, and caliper. Check that the wheel is fully seated in its dropouts or axle mounts, that the rotor runs through the caliper without obvious contact, and that the pads have usable material. A lever that pulls unusually close to the handlebar, feels spongy, or shows a fluid leak is not a rideable condition.
Do not put oil, grease, chain lubricant, or cleaning residue on a rotor or pad. Contamination can reduce friction even when the brake looks clean. Keep hands clear of the sharp rotor edge and allow hot parts to cool before inspection.
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Rubbing after transport
A rotor that rubs after a wheel has been removed or the bike has been transported commonly points to wheel seating or caliper alignment. Re-seat the wheel using the manufacturer’s axle procedure and check whether the rotor is visibly bent. Do not bend a rotor by hand or loosen caliper hardware unless the brake maker’s procedure specifically calls for an alignment adjustment.
Persistent rubbing, a rotor that is visibly deformed, or contact that worsens when the brake is applied warrants inspection by a qualified bicycle technician. Shimano’s safety guidance says a cracked or warped rotor must be replaced.
Squealing does not determine brake safety
Noise alone cannot tell you whether a brake is safe or unsafe. Assess pad thickness, rotor condition, contamination, wheel security, and lever feel. A squeal with clean, adequately thick pads and a straight rotor may require a manufacturer-specific bedding or alignment procedure; a squeal accompanied by oil contamination, glazing, heat damage, or weak braking requires parts or service rather than masking spray.
Weak braking and hydraulic concerns
Weak braking can result from contaminated or worn pads, a damaged rotor, poor wheel seating, air in a hydraulic system, or a mechanical adjustment problem. Shimano states that oil-contaminated pads or pads worn to 0.5 mm or less must be replaced. Do not try to wash oil out of a pad and return it to service.
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Brake limits documented by Shimano
| Component | Condition requiring replacement or action | Source qualification |
|---|---|---|
| Brake pad | Replace at 0.5 mm thickness or less; replace when oil has contaminated the pad or when the pad-retaining spring interferes with the rotor. | Shimano General Safety Information, 2018 manual. |
| Brake rotor | Replace if cracked or warped; Shimano safety guidance also cites 1.5 mm or less as the minimum rotor thickness. | Shimano General Safety Information, 2018 manual. |
Measure according to the brake manufacturer’s method. Do not infer pad or rotor limits from a different caliper family, because pad shape, backing plate, compound, and rotor specification vary.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Choosing replacement parts without creating a new fault
Cargo-bike parts are model-specific. Before ordering, identify the exact:
- Brake caliper and pad shape, including the backing-plate dimensions and the compound specified by the brake maker.
- Rotor diameter, mounting pattern, and minimum thickness specification.
- Battery model and capacity, lock or mount design, and charger compatibility.
- Motor or drive-unit version.
- Display and controller generation.
- Connector type and cable length.
The generic phrase “hydraulic disc brake pads for e-bike” is not enough to select a safe part. Match the exact pad shape and compound to the caliper. For Bosch and Tern battery, motor, and display components, the manufacturer manuals and an authorized service channel are the authority. A physically similar battery, display, or connector can still have incompatible firmware, voltage, communications, or safety electronics.
When to stop home troubleshooting
Arrange an authorized dealer or qualified bicycle service inspection when any of these conditions applies:
- The same electrical error returns after the specified power-down and connector checks.
- The battery shows a defect pattern, will not charge, becomes unusually hot, or has visible damage.
- A connector, cable, controller, or motor has heat discoloration, melted plastic, corrosion, or water intrusion.
- The motor cuts out repeatedly under ordinary load.
- The display remains blank after charge, seating, dry-contact, and shutdown checks.
- A brake lever reaches the handlebar, feels spongy, leaks fluid, or does not stop the wheel reliably.
- A rotor is cracked, warped, visibly damaged, or below the applicable minimum thickness.
- You cannot identify the exact component version required for a replacement.
Do not open a Bosch battery or motor. Internal electrical and hydraulic work should be performed by technicians with the appropriate tools, parts, and system documentation.
Post-repair control check and test ride
- Reinstall every cover, battery, wheel, and fastener according to the manufacturer’s procedure.
- With the bike stationary, switch on the system and confirm that the display, controls, lights, and assist selection respond normally.
- Squeeze both brake levers firmly. Confirm that each lever stops with a solid feel and does not pull to the bar.
- Spin each wheel by hand and listen for abnormal rubbing; verify that the wheel is securely seated.
- Check steering movement and make sure cables are not stretched, pinched, or rubbing the tire.
- Test in an empty, safe area at walking speed before carrying cargo. Apply each brake separately, then together.
- Stop immediately if braking, steering, power delivery, or display behavior is abnormal. Obtain professional service instead of continuing the test.
Bottom line
Begin with the basics that prevent damage: power off, secure the bike, seat the battery, and inspect clean, dry connectors. Record the exact code and system generation before searching for a remedy. Battery and motor internals are not user-serviceable on Bosch systems. For brakes, judge safety by pad and rotor condition, contamination, wear limits, wheel seating, and lever feel—not by noise alone. Confirm every replacement part against the exact caliper, battery, display, and motor version, then complete a stationary control check and a slow, unloaded test before returning the cargo bike to service.
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