What causes FANUC alarm 309?

Absolute pulse coder (APC) battery is dead or low, causing loss of position data and preventing reference return. Parameter settings for the absolute pulse coder are incorrect or have been reset, particularly parameters 1815 and 1816. The absolute pulse coder itself has failed or lost its stored position data due to power interruption. Bit 4 (APZ) of parameter 1815 has been cleared, indicating the zero point of the absolute pulse coder is not established. A mismatch exists between the stored absolute position and the actual machine position after a battery replacement or alarm.

How do I fix FANUC alarm 309?

1. Check the APC battery voltage on the affected axis. If the battery is low or dead, replace it before proceeding, as this is the most common cause of this alarm. 2. After replacing the battery, check parameter 1815 bit 4 (APZ) for the affected axis. If it is set to 0, the absolute zero point has been lost and must be re-established by performing a manual reference return. 3. Power the machine off and back on after any battery replacement, then attempt a manual reference return (ZRN) on the affected axis using the machine zero return mode. 4. If the alarm persists, verify parameters 1815 and 1816 for the affected axis are set correctly per the machine builder's parameter sheet. An incorrect setting here will block reference return. 5. If parameter corrections and battery replacement do not resolve the issue, the absolute pulse coder encoder may be faulty and require replacement. Check for any related encoder alarms on the same axis. 6. After a successful reference return, confirm the machine zero position is correct by checking the actual position against known reference points before resuming production.

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FANUC Controller

Servo/Axis · Intermediate · 30-90 minutes

Alarm 309

Fanuc Alarm 309 - APC ALARM: n AXIS ZRN IMPOSSIBLE

Alarm 309 occurs when the CNC detects an absolute pulse coder (APC/absolute encoder) on the specified axis but the system is configured to perform a reference position return (zero return). Absolute encoders maintain position without requiring zero return, so parameter settings must match the encoder type installed.

FANUCServo/AxisIntermediate30-90 minutes

309

Fanuc Alarm 309 - APC ALARM: n AXIS ZRN IMPOSSIBLE

SERVO ALARM 309

Root Cause Summary

Alarm 309 occurs when the CNC detects an absolute pulse coder (APC/absolute encoder) on the specified axis but the system is configured to perform a reference position return (zero return). Absolute encoders maintain position without requiring zero return, so parameter settings must match the encoder type installed.

Causes and Fixes

1
Navigate to SYSTEM > PARAM and check parameter 1815 for the affected axis. Bit 0 (APZx) should be set to 1 if an absolute encoder is installed, 0 for incremental encoder requiring zero return.
2
Verify the actual encoder type on the servo motor nameplate or amplifier label. Match αi series motors typically use BiS-C or serial absolute encoders; older α series may use pulse coder (incremental).
3
If absolute encoder is confirmed installed, set parameter 1815 bit 0 (APZx) to 1 for the affected axis. If incremental encoder is installed, set bit 0 to 0 and establish zero return method via parameters 1815 bit 1-3.
Parameter write enable must be activated (parameter 8900#0=1). Machine will require restart after parameter change.
4
After parameter correction, power cycle the CNC and servo amplifiers completely. Check DIAGNOSIS > SERVO > absolute position data screen to verify encoder battery status and position data validity.

DIY Feasiblehigh confidence

Call a technician if:

·Encoder replacement required
·Parameter corruption requiring full system backup restoration
·Mechanical repairs needed

Prevention

Monitor encoder battery status regularly
Perform periodic reference return verification
Keep encoder cables properly secured and protected
Maintain clean electrical connections

Common Mistakes

Manually changing P1820 without proper procedure
Ignoring encoder battery warnings
Forcing reference return with mechanical interference present
Mixing incremental and absolute parameter settings

Related Alarms

308 310 401 414

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