Fanuc Alarm 401 — Servo Axis Overload Fix

Error Code: Fanuc Alarm 401

What it means: Alarm 401 (SERVO ALARM: n-TH AX OVERLOAD) means the servo amplifier detected an overload condition on the specified axis. The drive is drawing more current than the rated continuous limit, the thermal model inside the amplifier calculated that motor temperature is exceeding safe limits, or an actual overtemperature signal was received from the motor’s thermistor. To protect the motor and amplifier from damage, the control shut down the affected axis.

This is a serious alarm that should not be repeatedly cleared and ignored — it indicates the servo system is working harder than it should, and ignoring it leads to motor or amplifier failure.

Common Causes

• Mechanical binding or excessive friction — A linear rail that’s dry, contaminated, or damaged causes the motor to work harder to move the axis. Ballscrew wear, loose ball nut preload, or a crashed axis with bent components all increase drag dramatically.
• Incorrect servo parameters — Servo gain parameters (especially current limit, CMR, and acceleration rate) set too aggressively can cause the drive to command excessive current during normal moves.
• Failing servo motor — A motor with failing bearings, damaged windings, or contaminated windings draws significantly more current than a healthy motor for the same load.
• Overloaded axis — cutting forces too high — In machining applications, taking too deep a cut or running feed rates too high causes excessive cutting force that the axis servo must push against. This is an application problem, not an equipment problem.
• Failing servo amplifier — An amplifier with degraded current sensing or a failing IGBT can report false overload conditions or genuinely overcurrent during normal operation.
• Ambient temperature too high in the cabinet — The servo amplifier’s built-in thermal model accounts for ambient temperature. A cabinet running at 45°C+ degrades the amplifier’s continuous current rating.

Step-by-Step Fix

1. Note which axis is alarming and when. Is it alarming at startup (before any movement)? During specific moves? Only during heavy cuts? The timing tells you everything: startup = mechanical binding or parameter issue; only during heavy cuts = application problem; any move = motor or amplifier fault.

2. Check axis for mechanical binding. With the machine in E-stop, manually push the affected axis (on machines with ballscrews, this requires disengaging the motor or using the jog pendant carefully). Smooth, light resistance = normal. Heavy resistance, grinding, or rough spots = mechanical problem. Check way lubrication (oil level, lube system pump operation), check for debris in the way covers, and inspect the ballscrew for damage.

3. Verify way lube system is operating. Most CNC machining centers have automatic way lube systems (Bijur or equivalent). Check the oil reservoir level and confirm the pump cycles correctly. Dry linear rails double or triple the servo load.

4. Check the motor thermistor signal. If the control has a servo status page showing motor temperature or thermistor state, check it. A thermistor reading at room temperature that shows “overheat” indicates a failed thermistor or wiring fault rather than a genuine overload.

5. Review servo load meter during operation. Most Fanuc systems have a servo load meter (PMC data, or via the Diagnosis screen). Run the axis and watch the load percentage. If the axis is at 80%+ load on a simple, unloaded jog move, the mechanical system is absorbing too much power. Healthy unloaded axes should show 5–20% load at typical jog speeds.

6. Check amplifier cooling. Inspect the servo amplifier cabinet cooling fans — confirm they’re running. Clean the cabinet air filter if present. Measure ambient temperature inside the cabinet. Above 45°C, amplifier continuous ratings degrade significantly.

7. Review and adjust servo parameters (with caution). If all mechanical and thermal checks are good, a Fanuc-trained tech should review the servo parameter file — specifically the current limit (parameter 2065), motor ID (parameter 2020), and acceleration/deceleration rates. Incorrect motor ID causes the amplifier to use the wrong current tables.

Parts That May Need Replacement

When to Call a Professional

If axis movement is smooth, lube system is working, cabinet temperature is acceptable, and you’re still getting Alarm 401 — this requires a Fanuc servo technician. Servo parameter diagnosis requires the specific Fanuc drive series documentation and hands-on access to the servo tune screens. A mismatched motor/amplifier combination (common after aftermarket repairs) will produce chronic 401 alarms that look like mechanical problems but are actually parameter issues. Tell the tech: “Alarm 401 on axis [n], mechanical system moves freely, way lube is working, cabinet is cool. I need servo parameter verification and load meter analysis.”

Pro tip: Before calling a tech, check the Fanuc alarm history for any preceding alarms. Alarm 401 is often the result of another event. If you see Alarm 414 (following error) or Alarm 445 (soft thermal) appearing just before 401, those upstream alarms tell the real story. A 401 that always follows a 414 points to following error → overcurrent → thermal — a mechanical or gain issue, not a motor failure.