grblHAL Alarm 20

grblHALAlarmNot a position problem

The controller lost contact with an I/O expander board.

Not a position problem. Hamrun's reading: this is about inputs and outputs. If it stopped mid-cut, home before trusting position. (Hamrun's reading)
What to do
  1. Check the cable to the expander board and its power.
  2. Reset the controller.
Official grblHAL message: “I/O expander communication failed.” 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.

Related codes

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.