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An electric cargo bike that will not start, cuts out under load, shows an error, or brakes badly should be diagnosed in a fixed order: make the bike safe, switch the system off, check battery seating and clean, dry connections, record the exact code and system generation, then escalate persistent electrical or safety-critical faults to an authorized dealer or qualified bicycle shop. Never open a Bosch battery or motor, bypass a brake cut-off, or continue riding when braking or steering is uncertain.
Start with a safe diagnostic setup
Cargo bikes are heavy, and a loaded bike can move unexpectedly while you are working on it. Put it on stable ground, secure or remove the cargo, and keep hands, clothing, and tools away from moving wheels. Rotors and calipers may be hot after riding, and a rotor edge is sharp.
- Switch the electrical system off before inspecting connectors.
- Remove the battery only when the manufacturer’s procedure calls for it, and support the pack so it cannot fall.
- Do not bypass a brake cut-off, short contacts, or force a connector.
- Do not ride for a “quick test” if a lever pulls to the handlebar, a rotor is visibly damaged, or steering and wheel seating are uncertain.
Write down the bike model, battery and motor generation, display model, and the exact wording or number of any error. A code is not universal: the same number can mean something different on another brand or system generation.
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| Symptom | First checks | Stop riding and obtain service when |
|---|---|---|
| Nothing happens when you press the power button | Battery charge, pack seating and lock, contacts, display seating, and display cable | The battery or motor may have an internal fault, a connector is hot or damaged, or the system still will not start after a complete shutdown and correct reinstallation |
| Battery LEDs behave abnormally | Temperature, charge level, and visible contamination or damage at the contacts | LEDs 2 and 4 flash, the pack is damaged, or charging remains unavailable in the correct temperature range |
| Motor cuts out on a hill or with a heavy load | Record the code, power off, remove the battery, discharge residual power, and inspect battery, controller, and motor connectors | Cutouts repeat, connectors are hot or wet, or the fault returns after the manufacturer reset procedure |
| Display is blank | Battery charge, display seating, clean and dry mount contacts, display battery if fitted, and cable alignment | The display remains dark after a full shutdown or shows a persistent error |
| Brakes rub, squeal, or feel weak | Wheel seating, rotor alignment, pad thickness and contamination, and stationary lever feel | The lever reaches the bar, fluid leaks, the rotor is cracked or warped, or braking remains uncertain |
Battery will not charge or the bike will not turn on
1. Confirm the pack is actually seated
Turn the system off. Check that the battery is fully inserted, locked, and sitting flush in its mount. A pack that is almost in position can appear secure while its contacts do not make a reliable connection. Do not hammer or twist the battery into place; remove it and reinstall it along the guide rails according to the bike manual.
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Inspect the battery contacts and the matching connector for dust, moisture, corrosion, bent parts, or impact damage. They must be clean and dry. If you find liquid, allow the area to dry completely before reconnecting; do not probe contacts with metal tools or spray cleaner into the socket.
2. Read the battery indicators
Tern’s Bosch troubleshooting guide documents these battery indications. They are useful clues, not a substitute for the manual for your exact battery generation.
| Indicator behavior | Meaning reported by Tern | What to do |
|---|---|---|
| LEDs 1, 3, and 5 flash | The battery is outside the 0–40°C (32–104°F) charging range | Move the pack to a location within that range and allow it to stabilize before charging |
| No LEDs when switching on | About 5% charge or less | Connect the correct charger and check whether charging begins |
| LEDs 2 and 4 flash | The battery has detected a defect | Stop troubleshooting at the connector level and contact an authorized dealer or Bosch service center |
The temperature range is a charging condition, not permission to heat or cool a battery aggressively. Do not place a pack on a radiator, in a freezer, or in direct sun to force it into range.
3. Check the charger path without opening anything
Use the charger specified for the battery and verify that its plug is fully inserted at both ends. Look for crushed insulation, a bent plug, or a damaged socket. If the charger or battery becomes unusually hot, smells burnt, or shows physical damage, unplug it and arrange professional service. Never open the pack to test cells or replace an internal fuse.
4. Reset the system correctly
If the pack and contacts look normal, Tern recommends a complete shutdown, removing the battery for a few minutes, and reinstalling it correctly. After reinstalling, attempt one normal start. Repeatedly cycling a suspected fault or repeatedly reconnecting a wet plug can worsen contact damage.
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If the bike still will not start, use the model-specific manual and contact an authorized Tern dealer or Bosch service center. Tern’s explicit warning is: “Do not open the Bosch motor or battery.” That restriction matters because both contain high-energy components and sealed electronics that require specialized diagnostic equipment.
Motor cut-outs and electrical error codes
Why a motor can stop under load
A cut-out on a steep hill or with an unusually heavy cargo load can be a protection response, but it can also indicate a poor connection or a failing component. 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. Its guidance notes that the code can appear when the bike is pushed harder than recommended, such as on a steep hill or with excessive load.
Do not assume that Error 21, or any other number, has the same meaning on a Bosch, Shimano, Rad, or another system. Identify the exact drive-system generation first, then use that manufacturer’s owner’s manual or app.
Manufacturer-style reset and connector inspection
- Power off. Stop pedaling, switch the system off, and secure the bike.
- Remove the battery. Follow the lock and removal procedure for your model; do not pull on wiring.
- Discharge residual power. Rad Power Bikes instructs owners dealing with Error 21 to press the MODE/power button after battery removal to discharge remaining power.
- Inspect the battery connection. Check the battery terminals and the bike-side connector for dirt, moisture, corrosion, heat discoloration, bent contacts, or looseness.
- Inspect the motor connection. Locate the motor connector using the bike manual. It must be clean and dry and free of crushed or stretched cable.
- Reconnect straight. Pull connectors apart and reconnect them without twisting. Align any keying marks; never force a plug that does not seat easily.
- Reinstall and test once. Lock the battery in place, power up, and check whether the code returns before attempting a ride.
A connector that is hot, melted, wet inside, or repeatedly loose is not a cleaning problem. Stop and use qualified service. Water intrusion, an internal battery fault, and motor or controller faults require equipment and procedures that are not safe to improvise at home.
Fixing a blank or failing display
When the screen is completely dark
A dark display can result from a depleted main battery, a display that is not seated on its mount, dirty or wet mount contacts, an empty display battery where the design includes one, or a cable loosened during transport. Work through those possibilities with the system powered off.
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- Confirm the main battery has charge and is locked into the frame.
- Remove and reseat the display on its mount, if the design allows removal.
- Inspect the mount and contacts for dust, moisture, corrosion, or bent pins; clean only as the manufacturer permits and let moisture evaporate.
- If the display has its own battery, check that battery according to the display manual.
- Follow the display cable from the mount toward the controller, looking for a loose plug, incorrect alignment, pinched insulation, or damage caused by folding or transporting the bike.
- Perform the full shutdown: switch off, remove the main battery for a few minutes, reinstall it, and try one restart.
Do not pull a cable by its wire or rotate a keyed connector to make it fit. If the screen remains blank, the fault may be in the mount, controller, wiring, or display electronics.
When the display shows an error
Photograph the code before cycling power. Record whether it appeared at startup, during charging, under motor load, or after transport. Then look up the code in the exact Bosch, Shimano, or bike-manufacturer manual for that generation. Tern publishes separate GSD documentation for Bosch batteries, chargers, drive units, and displays; selecting the wrong generation can lead you to the wrong connector layout or reset procedure.
A persistent code, a code accompanied by heat or water intrusion, or a system that loses assistance repeatedly belongs with an authorized dealer or qualified service center. Do not clear a code merely to make the display look normal and then continue riding.
Brake rubbing, squealing, and weak braking
Begin with a stationary control check
With the bike stationary, squeeze each brake lever firmly. The lever should not pull to the handlebar, feel spongy, or continue moving toward the grip. Check that each wheel is fully seated in its dropouts or axle mounts and that the rotor runs through the caliper without obvious contact. A rotor that started rubbing after transport can point to wheel seating or caliper alignment rather than worn pads.
Noise alone does not determine brake safety. Inspect pad thickness, rotor condition, contamination, and lever feel. A quiet brake can still be unsafe, while a brief squeal does not automatically mean the brake has failed.
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Pad and rotor limits
| Component | Replacement condition | Source and qualification |
|---|---|---|
| Brake pad | Replace at a thickness of 0.5 mm or less; replace if oil has contaminated the pad or if the pad presser spring interferes with the rotor | Shimano General Safety Information, 2018 manual |
| Brake rotor | Replace at 1.5 mm or less, or immediately if cracked, warped, or otherwise visibly damaged | Shimano General Safety Information, 2018 manual |
Measure pads and rotors with the method specified by the brake maker. Do not count the backing plate as friction material, and do not continue using a rotor with a crack or deformation. Rotor and caliper surfaces can be hot after braking.
Contamination and cleaning mistakes
Keep oil, grease, chain lubricant, and cleaning sprays away from the rotor and friction material. Shimano states that oil-contaminated pads must be replaced; trying to wash oil out of a pad does not restore predictable friction. If a rotor is contaminated, use only a cleaning method approved in the brake manual. Do not sand a visibly damaged rotor or compensate for contamination by squeezing the lever harder.
When alignment or hydraulic work is needed
Persistent rubbing after the wheel has been correctly seated can require caliper alignment or inspection for a bent rotor. A lever that reaches the bar, visible fluid, recurring loss of pressure, or uncertain braking requires professional inspection. Hydraulic bleeding and caliper service must use the brake maker’s specified fluid, bleed procedure, and replacement parts; mixing fluids or guessing at a bleed kit can damage seals.
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Confirm the exact component before ordering
Cargo-bike parts are model-specific. Record the bike model and year or generation, battery capacity and part number, motor and display model, brake caliper model, and pad shape before buying anything. A listing described only as “hydraulic disc brake pads for e-bike” is not enough: the pad outline, backing plate, thickness, spring, and compound must match the caliper.
Use the manufacturer manual as the authority for Bosch and Tern battery, motor, charger, and display parts. For a brake part, identify the caliper marking and compare it with the brake maker’s parts diagram. Do not substitute a rotor simply because its diameter appears correct; thickness, mounting pattern, and system approval also matter.
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What you can reasonably check yourself
- Battery lock and seating, visible contact cleanliness, and charge indicators.
- Display seating, mount condition, and visible cable alignment.
- Wheel seating, rotor damage, pad thickness, and stationary lever feel.
- Recording an error code and matching it to the exact system manual.
What belongs with a professional
- Any internal battery, motor, controller, or charger repair.
- A battery-defect indication, repeated electrical cut-outs, water intrusion, or heat-damaged connector.
- Hydraulic bleeding, persistent pressure loss, leaking lines, or a lever that pulls to the bar.
- Cracked or warped rotors, damaged mounts, or any fault that leaves braking or steering uncertain.
An authorized dealer, Bosch service center, or qualified bicycle service shop can identify the correct generation, read system diagnostics, and fit approved parts. The correct escalation is part of the repair, not a failure to troubleshoot.
Post-repair control check and safe test ride
Before moving the bike, verify that the battery is locked, the display and cables are seated, wheels are secure, and no tool or loose cable can contact a wheel or rotor. Squeeze both brake levers while stationary and confirm normal, firm travel. Spin each wheel by hand only when it is safe to do so, listening for persistent scraping and checking that the rotor is not visibly damaged.
Begin the test in a quiet, level area without cargo or passengers if the bike can be safely unloaded. Use the lowest practical assistance, ride slowly, and apply each brake separately before increasing speed. Stop immediately if the motor cuts out again, an error returns, braking feels weak or uneven, steering changes, or a connector becomes hot. Do not turn a successful power-on into a full-load trip until the bike has passed this controlled check.
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- Keep battery, display, and motor connectors clean and dry whenever the battery is removed or the bike is transported.
- Secure the display and cables before folding, loading, or carrying the bike so plugs cannot be pulled sideways.
- Inspect brake pads, rotors, wheel seating, and lever feel before carrying a heavy load.
- Keep the exact owner’s manual and system-generation information with the bike, rather than relying on a generic error-code list.
- Record recurring codes, the load and terrain when they occur, and whether the battery was hot or wet; that information helps a technician reproduce the fault.
No single fault can be called the statistically most common from the available manufacturer guidance. The practical priority is safety: eliminate simple seating, charge, temperature, contamination, and alignment causes first, then stop escalating when the evidence points to sealed electronics or compromised brakes.
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