How to Set the Reference Position (Zero Return) on a Fanuc CNC — 0i / 16i / 18i / 21i / 30i

If your Fanuc control shows APC alarms (SV0300 / 300–308), asks for zero return after a battery change, or has lost its position after an encoder or motor swap, the machine reference (zero point) must be re-established before you can run production. This guide covers the two cases you will meet on Fanuc 0i, 16i, 18i, 21i and 30i series controls: absolute pulse coder mastering (most machines today) and dog-type reference return (older or retrofitted machines).

Before you start

• Clear any obstacles from the table and tool changer area — the axes will move.
• Have the machine's axis reference data sheet from the builder if available (grid shift / reference offset values).
• If the position was lost after a crash or mechanical work, first confirm the axis moves freely in handle mode.

Method 1 — Absolute pulse coder mastering (dogless)

1. Put the control in MDI mode and press OFS/SET → SETTING. Set PWE = 1 (parameter write enable). Alarm SW0100 appears — this is normal.
2. Press SYSTEM and open parameter 1815.
3. For the axis to master, set bit 4 APZ = 0. This tells the control the zero point is not established.
4. In JOG or HANDLE mode, move the axis to the machine's reference position. Use the builder's alignment marks, a dial indicator on the reference block, or the recorded distance from the datum.
5. Return to parameter 1815 and set APZ = 1 for that axis. The current position is now stored as the reference.
6. Set PWE = 0, then power the control OFF and back ON.
7. Verify: the machine coordinate should read zero (or the builder's grid shift value) at the reference mark. Check soft overtravel limits by jogging slowly toward each end of travel.

Method 2 — Dog-type reference return

1. Select REF (ZRN) mode on the operator panel.
2. Press and hold the axis direction key (usually +X, then +Z on lathes; +X/+Y/+Z on mills). The axis moves at rapid until it strikes the deceleration dog, slows, backs off, and creeps to the next encoder grid point.
3. The reference-established LED lights and the machine coordinate resets.
4. If the axis stops short or overshoots, check parameter 1425 (FL creep speed) and the condition of the deceleration dog and limit switch.

Common problems

• SV0300 needs power off after APZ set — normal; cycle power once.
• Zero point is off by exactly one grid — adjust grid shift parameter 1850 by one motor revolution's worth of pulses, or re-master more carefully.
• APZ will not stay at 1 — encoder battery voltage low (check alarm APC 306/307), cable or pulse coder fault. Replace the battery first; if it repeats, the pulse coder or feedback cable is failing.
• Position lost again after power off — the 6 V encoder battery pack is dead or wiring to the drive amplifier battery input is broken.

Related guides

• How to replace a Fanuc CNC battery
• How to replace the pulse coder on a Fanuc servo motor
• Fanuc Series 0 absolute referencing procedure