ER21 - Sensor error within the print head

This error can show up during the active leveling process. It means that there is a problem with the capacitive sensor of the print head, which is used for the active leveling.

There are multiple components involved in the active leveling process, which can make troubleshooting tricky. For detailed advice on dealing with leveling problems, please visit this page:

Active leveling errors

For the ER21 message specifically, this is usually not caused by material stuck on the nozzle or build plate, or an incorrect height of the build plate (caused by manual leveling). ER21 is often a connection or communication problem. Check the following basic tips:

Don't touch the printer While the Ultimaker 3 is going through the active leveling process, do not touch the printer or the print head. This can interrupt the process and corrupt the measurements. Also, ensure that no other machinery nearby could be causing interference.
Check the wires

Open the print head fan bracket and inspect the two (red and white) wires at the back. Ensure that both wires are still securely connected to the connectors marked 'SHIELD' and 'SENSOR', and that the wires don't show any signs of damage.

Capacitive-sensor-wires-ok.jpeg Capacitive-sensor-wires-broken.jpeg
OK: Both wires are securely connected to the board and no damage is visible Not OK: One of the wires is broken and disconnected from the board
Check the connection

At the back of the print head, ensure that the connector for the capacitive sensor board is still securely connected to the print head PCB. This is the second connector from the left, as seen from the back, with the red and white wires.


Note: If problems persist after taking the steps on this page, contact support or your local reseller for further assistance. Ensure to include the error number and the log files of your printer.

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