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Troubleshooting

Ride1Up Core-5, 500 Series, 700 Series and LMT'D: error 21 or power cuts out on long climbs

Error 21 or a sudden cutout on a long Ride1Up climb is usually battery voltage sag or the controller protecting itself. Stop, wait 5 minutes, pedal more, and check the battery before suspecting hardware.

Last edited Nov 9, 2026 by Pedalwise team Edit History Add a subpage

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You're climbing a long hill, often on the throttle or a high assist level, and the motor suddenly cuts out. Error 21 may show, or the bike may simply lose power or shut off. This page covers Ride1Up models where 21 is a voltage current error: the Core-5, 500 Series, Cafe Cruiser, Turris and Portola with the KD21C display, and the 700 Series and LMT'D with the LMT'D display (KD21C codes, LMT'D codes). Short answer: the battery couldn't hold its voltage under that load, or the controller protected itself from too much current. Pull over, switch off, wait 5 minutes and switch back on (KD21C codes).

If the battery is hot, swollen, damaged or smells odd, stop using it and contact Ride1Up or a pro. Never open the battery or controller. See e-bike battery care.

Most likely cause

Error 21 means the battery isn't supplying enough voltage to the display and motor, and overworking the controller, motor or battery, such as long climbs without much pedaling, can cause it (KD21C codes). Here is why: under hard acceleration or on hills, battery voltage drops for a moment because of the battery's internal resistance. If it drops far enough, the controller cuts motor power until it recovers. Sag is worse on smaller, older, heavily used or low batteries, and at high assist levels (voltage sag).

Other causes to rule out

  • An aging battery. On the Core-5, an older battery that has been through many charge cycles may need replacing if 21 keeps appearing (Core-5 manual).
  • Low voltage. On the LMT'D display, code 26 is low voltage. Check the battery voltage and terminals (LMT'D codes).
  • Hardware. If 21 persists even on easy riding, the likely parts are the internal controller first, then the motor, then the battery, then the motor to controller wire if the connections look damaged (error 21).

How to confirm

  1. Pull over, turn the bike off, wait 5 minutes and turn it back on (KD21C codes).
  2. If the battery is low, drop to assist level 1 and keep riding. If the bike shuts off even on level 1, don't keep switching it back on. Go home and charge (voltage sag).
  3. Wait a few minutes after stopping before you read the battery level, since the reading recovers after load (voltage sag).
  4. Reset the display, then remove the battery, inspect its terminals and reinstall it gently (error 21).
  5. Note whether 21 only shows on long climbs or also on flat, easy rides. Climbs only suggests load and battery, while every ride suggests hardware.

Fix options

Ride with more pedal input on climbs (easy)

Error 21 comes with long climbs with little pedaling (KD21C codes). Sag is also more common at high assist levels (voltage sag).

Lower the current limit (easy, KD21C bikes)

On the 500 Series and Core-5 display, set maximum current to no more than 18A if you climb steep hills often (500 Series and Core-5 display). Follow that article for the menu steps.

Replace an aging battery (easy swap)

Ride1Up sells replacement batteries for many models, including a shared LMT'D V1 and Core-5 battery and a 500 Series battery (Ride1Up parts). Only use the battery made for your model.

Diagnose the controller or motor (pro or Ride1Up support)

If 21 shows on easy rides with a healthy battery, test with a matching bike or contact Ride1Up support with photos of the wires and connections (error 21, advanced troubleshooting).

Parts

  • E-bike battery (the Ride1Up battery for your model)
  • Controller (only after diagnosis)

Videos

700 Series, Rift & LMT'D Display Tutorial from Ride1Up: Ride1Up's guide to the buttons and settings menu on the 700 Series and LMT'D display, handy for the reset to defaults step for error 21.
How to Use the KD21C Display on Ride1Up Electric Bikes from Ride1Up: Ride1Up shows the buttons and settings on its KD21C display, which several of its other models use.

When to get a pro

Sources

Stuck after these checks? Find a shop near you.

Need a pro?

If these steps didn't fix it, find a shop near you in our partner network. For battery or wiring trouble, look for shops marked Battery and electrical.

Talk

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