Error 35 on the NIU KQi kick scooters (KQi2, KQi3, KQi3 Max) means “Batter over-temperature warning.” Most likely cause: the battery is outside its safe temperature range. Try this first: bring the scooter to room temperature.
The battery protects itself when it's too hot or too cold to charge or ride safely. The code should clear once the battery is back at a normal temperature.
Step-by-step checklist
Work from the top. Steps go from free and easy to shop jobs. Switch the scooter off before touching any connector. Tick each step as you go. Your progress is saved on this device.
When it's time for a shop: if you've worked through every step and the problem is still there, contact the brand's support or an authorized service center. Bring your notes on when it happens, plus the code.