Fanuc alarm DS0309: APC ALARM: REF RETURN IMPOSSIBLE

What it means

An attempt was made to set the absolute-position detector zero point by MDI operation when the zero point could not be set — usually because the detector count has no valid correspondence within one revolution of the shaft. Rotate the motor manually at least one full revolution, then cycle CNC and servo amplifier power before setting the zero point again. This is a procedure error rather than a hardware fault in most cases.

Likely causes

• Zero-point setting attempted without establishing a one-revolution correspondence
• Motor positioned exactly at the grid boundary during the attempt
• Residual effect of a prior battery-zero or pulsecoder replacement