Error 25 on the 850C means “Torque sensor error (torque).” Most likely cause: a loose or wet connection, or a sensor fault. Try this first: turn it off and on again.
Torque sensors measure how hard you push on the pedals and match the motor's help to it. This code means that signal is missing or out of range. Check the connections, then leave the sensor to a shop.
Step-by-step checklist
Work from the top. Steps go from free and easy to shop jobs. Switch the bike off before touching any connector. Tick each step as you go. Your progress is saved on this device.
Parts you might need
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Only buy a part if the free steps didn't fix it. Always match the voltage and connector type to your bike.
Electrical contact cleaner spray
Cleans dirt and corrosion from connector pins and battery contacts.
When it's time for a shop: if you've worked through every step and the problem is still there, take the bike to an e-bike shop. Bring your notes on when it happens, plus the code and your error history.