Fanuc absolute encoder battery
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Fanuc Alarm 800: APC Alarm
Alarm Message: APC ALARM 800
Full Display: 800 APC ALARM: X AXIS or similar per axis
Affected Systems: Fanuc 0i, 16i, 18i, 30i, 31i, and others with absolute encoders
The 800-series APC alarms relate to the Absolute Pulse Coder (APC) — Fanuc’s absolute position encoder. These alarms indicate the encoder cannot communicate position data to the servo amplifier.
APC Alarm 800 Sub-Codes
| Alarm | Meaning |
|---|---|
| 300 | APC alarm: battery voltage 0 |
| 302 | APC alarm: communication error |
| 303 | APC alarm: absolute position lost |
| 304 | APC alarm: absolute position not set |
| 305 | APC alarm: rotation count lost |
| 307 | APC alarm: battery voltage too low |
| 360 | APC alarm: APC fault |
Most Common Cause: Battery Failure
APC encoders use a backup battery to maintain position data when the machine is powered off. When this battery fails:
- Alarm 300 or 307 — battery voltage low or zero
- After replacing the battery, the machine must be re-referenced (zero return)
Battery location: Usually in the electrical cabinet or on the servo amplifier itself. On some machines, each axis has its own battery. Check your machine tool builder documentation for exact location.
Battery type: Fanuc uses A98L-0031-0005 (3.6V lithium) or similar. Always replace with the specified Fanuc battery — third-party batteries may cause re-alarm.
Alarm 302 — Communication Error
The APC serial communication between the encoder and the servo amplifier is interrupted. Check:
- Encoder cable continuity — from encoder to amplifier
- Connector seating at both ends (encoder and amplifier)
- Cable shielding — a broken shield allows EMI interference
- Servo amplifier encoder interface — check for damage on the port
Alarm 303/304 — Position Lost / Not Set
The encoder lost its absolute position reference. This occurs after battery failure. After replacing the battery, perform a zero return (reference return) on the affected axis per your machine’s procedure. The position is re-established on reference return.
Zero Return Procedure (After Battery Replacement)
- Replace battery with machine power OFF
- Power on machine
- Acknowledge the APC alarm
- Perform manual zero return (REF operation) on all affected axes
- Confirm position display matches expected machine zero
Parts Often Needed
| Part | Notes |
|---|---|
| Fanuc A98L-0031-0005 battery | 3.6V lithium, use Fanuc OEM |
| Encoder cable | Replace if comm error persists after battery |
| Servo amplifier (SVM) | Only if interface card confirmed damaged |
Technician Tips
Fanuc CNC alarms use a prefix system that tells you where the fault lives: SV = servo, SP = spindle, PS = PMC/PLC, OT = overtravel. SV041 (position error too large) is the most common servo alarm — check the servo amplifier LED before replacing the motor or encoder. If the amplifier shows a ”—” or blank display, the issue is likely the power supply to the amplifier. If the display is normal, use the servo monitor screen (function key + SV toggles) to check actual vs commanded position. A 0.1mm following error at rest is normal; anything over 0.5mm at standstill means the axis is drifting.
Cost guide: Fanuc servo amplifier replacement runs $2,000-$4,000. Encoder cable replacement: $200-$400. Always try cable reseating first — it fixes about 30% of SV alarms.
Need Replacement Parts?
Before ordering, match the machine-control part to the model number, electrical rating, and failure symptom. Check current prices on Amazon - prices and availability change frequently.
Jump to Fix
- Battery alarm (300/307) → Replace battery → Re-reference axes
- Comm error (302) → Inspect cable → Check connectors → Re-reference
- Position lost (303/304) → Replace battery if needed → Perform zero return
When to Call a Pro
APC encoder replacement on servo motors requires motor removal and precise assembly. Contact your machine tool builder or a Fanuc-certified service provider.