grblHAL Alarm 21

grblHALAlarmNot a position problem

The controller couldn't read or write its settings memory.

Not a position problem. Not a motion problem. (Hamrun's reading)
What to do
  1. Reset the controller and check your settings ($$) are still right.
  2. If it repeats, the settings memory may be failing.
Official grblHAL message: “Non Volatile Storage (EEPROM) failure.” source

Got a different message?

Paste it into the decoder. Hamrun's editor checks your G-code before you run it, so a toolpath that leaves the machine's travel or a missing feed rate is caught before the machine stops.

Official text copied from the grblHAL source on 2026-11-01. Steps and the position answer are Hamrun's reading unless marked “firmware says”. User reports are labelled CLAIMED.