- Electric mobility
- Electric scooter
Error 21
Xiaomi Error 21 on electric scooter: controller and battery not communicating
Risk level
Medium
Proceed carefully and stop if the check is unclear.
Urgency
Medium
Quick answer
Xiaomi Error 21 means the controller cannot communicate with the battery. It often follows a battery firmware update or a non-original battery. Do not ride or open the battery: Xiaomi refers you to its after-sales service.
Safety first
Stop and check this first
- Do not open the battery or connect batteries from other models: Xiaomi names third-party batteries as a cause of Error 21. If the battery is swollen, deformed or smells, stop riding and charging the scooter and take it to authorized service.
Safe checks you can perform
- Battery origin
- Recent update in the app
- Error details in the app
Leave these tasks to a qualified professional
- Do not attempt this professional task: Battery communication test
- Do not attempt this professional task: BMS board or battery diagnosis
What a technician must be able to justify
A technician is coming
These are the steps they must carry out and show you. Tick them as they work: if steps are skipped, do not accept a part swap.
These are not instructions for opening or handling the equipment yourself. They are the checks that should support a technical conclusion.
Battery communication test
What they doAuthorized service checks the communication harness between battery and controller and its terminals with the scooter open.
They must show youthe damaged cable or terminal, or that they are fine.
In writingwhich part was replaced (harness, controller or BMS) and why.
BMS board or battery diagnosis
What they doIf cable and controller are fine, the technician diagnoses the battery's communication circuit and replaces the BMS board or battery.
They must show youthe BMS diagnosis.
In writingthe reference of the battery or board fitted and the function test.
Before accepting a part or repair
Nothing has been shown yet. Do not accept a part swap based on the code alone.
Checks and evidence, step by step
Follow the documented order. An error code identifies the affected system, but it does not prove by itself which part has failed.
- 1
Battery origin
No special tools
Procedure
- Who:
- you.
- How:
- check whether the scooter's battery has ever been replaced and whether it is the original for the same model, using the invoice or repair report. Do not open the deck.
- Result that matters:
- whether there is a battery from another model or a third party.
- If there is:
- it is the cause Xiaomi names; the official fix is a same-model battery.
- If it is original:
- check for a recent update.
- Evidence to keep:
- invoice or report for any battery change.
Evidence: Xiaomi Electric Scooter 6 Pro Error code and repair guideline
- 2
Recent update in the app
No special tools
Procedure
- Who:
- you.
- How:
- in Mi Home, open the scooter card and check the firmware version and whether an update ran just before the error.
- Result that matters:
- whether Error 21 lines up with an update.
- If it does:
- tell the service; Xiaomi lists it among the causes.
- If there was no update:
- suspicion moves to the wiring, controller or BMS.
- Evidence to keep:
- a screenshot of the firmware version and date.
Evidence: Xiaomi Electric Scooter 6 Pro Error code and repair guideline · Xiaomi Electric Scooter 5 Plus FAQ
- 3
Error details in the app
No special tools
Procedure
- Who:
- you.
- How:
- connect the scooter to the Mi Home/Xiaomi Home app; Xiaomi says the error shows on the home page and tapping it opens the detailed description.
- Result that matters:
- that the app confirms Error 21 and not a similar number.
- If the app shows the same code:
- contact after-sales service, as Xiaomi advises.
- If it shows another code or none:
- note the one it shows, because it changes the diagnosis.
- Evidence to keep:
- an app screenshot with the code, the date and the exact scooter model.
Evidence: Xiaomi Electric Scooter 5 Plus FAQ
Where this code applies
A code is meaningful only inside the right product and version context.
Primary scope
- System
- Electric mobility
- Brand
- Xiaomi
- Product type
- Electric scooter
Known code variants
- 21
- error 21
- Error 21 height sensor
- Height sensor has been pressed
- iPhone error 21
- iTunes error 21
- Mi Electric Scooter error 21
- Ninebot codigo 21
- Ninebot error 21
- patinete Segway error 21
- patinete Xiaomi error 21
- Roborock Error 21
- Segway error 21
- Segway Ninebot E2 Pro error 21
- Segway Ninebot error code 21
- Xiaomi bateria error 21
- Xiaomi error 21
- Xiaomi error 21 6 km/h
- Xiaomi scooter error 21
What it means
Xiaomi defines code 21 as a communication failure between controller and battery. The controller queries the battery management system (BMS) and, without a valid reply, restricts the scooter; Xiaomi's support page ties it to a 6 km/h speed limit. The official causes are a damaged communication cable, a battery firmware update or a third-party battery. The fix ranges from the harness to the controller, the BMS board or the battery, so the code does not single out a part.
Warnings and stop conditions
- Higheditorial warning
Do not open the battery or connect batteries from other models: Xiaomi names third-party batteries as a cause of Error 21. If the battery is swollen, deformed or smells, stop riding and charging the scooter and take it to authorized service.
Evidence: Xiaomi Electric Scooter 6 Pro Error code and repair guideline
Symptoms and causes
Probable causes · in order
- Battery of another model or third partyCommon
Xiaomi names it outright: fixed by going back to a same-model original battery. Often appears right after a battery swap.
Evidence: Xiaomi Electric Scooter 6 Pro Error code and repair guideline · Xiaomi Electric Scooter 5 Plus FAQ
- Battery firmware updatePossible
The error starts after an update; Xiaomi lists it among the triggers for code 21.
Evidence: Xiaomi Electric Scooter 6 Pro Error code and repair guideline · Xiaomi Electric Scooter 5 Plus FAQ
- Damaged cable, controller or BMSPossible
No battery swap or firmware change. Xiaomi replaces the harness, controller or BMS board depending on where the damage is.
Evidence: Xiaomi Electric Scooter 6 Pro Error code and repair guideline · Xiaomi Electric Scooter 5 Plus FAQ
Virtual technician
Does Error 21 still appear after these checks?
Answer a few questions to narrow down the likely cause and the safest next action. Guidance only - it never replaces a professional.
Sources and technical references
- 1
Xiaomi Electric Scooter 6 Pro Error code and repair guideline
Xiaomi · 2026 · official_doc
- 2
Xiaomi Electric Scooter 5 Plus FAQ
Xiaomi · 2026 · official_doc
- 3
Mi Electric Scooter error 21 alarms and speed limit 6km/h
Xiaomi · 2026 · official_doc
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