The flow sensor has triggered

The Ultimaker S5 and the Ultimaker S3 feature a flow sensor in the feeder. The flow sensor is active during printing and measures how much filament has passed through the feeder. It can detect forward movement and retractions. The flow sensor readings are used to detect the end of the filament and severe underextrusion.

How the flow sensor works

As the filament passes through the feeder, the flow sensor triggers a type of reading. This reading is material specific, and can be correlated to the expected filament steps.

There is an expected amount of filament flow during printing. This flow is determined by the print core size, the filament type, and the print settings. There is an allowed deviation from the expected flow. This can compensate for occasional slippage, for example on soft materials such as TPU 95A.

However, if the deviation of the measured flow from the expected flow is too much, the flow sensor will trigger and a warning will be displayed. This will pause the print.

Note: The flow sensor is always disabled for the first two layers of a print job. This is because the pressure from the build plate on the nozzle would otherwise trigger too many false positives. If no material is extruding (due to a blockage or insufficient material) at the start of a print, it is normal that the warning will only appear on layer 3.

What actions to take

If the flow sensor is triggered because the material spool is empty, simply unload the material and replace it with a new spool. If the material is not empty but it is tangled on the spool, you can also unload it via the menu, untangle it, and reload the material to continue your print.

If there are no visible problems with the material spool, check the tips below:

Check for grinding Unload the material and check for signs of grinding on the filament. Remove the damaged part. It is highly recommended to also clean the feeder and the Bowden tube to remove filament debris if grinding is observed.
Clean the print cores Underextrusion and grinding are often caused by degraded or carbonized material in the nozzle. Clear any blockages by cleaning the print core(s) with the hot and cold pull method.
Filament quality Ensure the material you are using is of good quality. Only use filaments from reliable manufacturers to prevent seeing deviations in the filament diameter. Material that has absorbed moisture is also prone to cause extrusion problems. Try drying the material.

Note: It is not possible to perform maintenance actions such as cleaning or replacing the print cores during an active print job. This would cause reliability problems when continuing the print. If the flow sensor triggers because of extrusion problems, first abort the print.

Disable flow sensors

If the error is persistent while there is no end of filament or extrusion problem, this can be a false positive. It is recommended to contact support for further assistance. Please include your log files and information about the print job, including material types.

In the meantime, you can bypass this error by temporarily disabling the flow sensors in the menu. This will allow you to continue printing while waiting for additional support. To disable the flow sensors, click on the gear icon, then select Settings. Toggle the switch for the flow sensors to turn them off.


Note: If you are seeing print quality problems such as underextrusion in your print with the flow sensors disabled, the encountered errors were valid and not false positives.

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