Fanuc CNC Alarm 402 Cause and Fix

A Fanuc Alarm 402 is a common but critical error that indicates a Servo Alarm: SV Overload. Essentially, the CNC controller has detected that the electrical current being drawn by a specific axis motor is exceeding its rated capacity for too long.

The system triggers this alarm as a protective measure to prevent the motor or the servo amplifier from burning out. Here is a breakdown of what causes this alarm and how to systematically fix it.

The 402 alarm is usually followed by a specific axis letter (e.g., 402 X-AXIS OVERLOAD). This tells you exactly where the physical resistance or electrical failure is occurring. The causes generally fall into three categories:

Mechanical Binding or Obstruction

This is the most frequent culprit. If the machine’s moving parts are stuck, the motor has to work much harder to move the axis, causing an amperage spike.

• Lack of Lubrication: If the ways or ball screws are dry, friction increases significantly.
• Way-Wiper Interference: A damaged or loose wiper can jam against the guide rails.
• Hard Crashes: If the machine recently crashed, components may be misaligned or the ball screw may be bent.
• Heavy Loading: Attempting to take a cut that is too deep or using a dull tool can overload the spindle and the feed axes simultaneously.

Electrical or Parameter Issues

• Short Circuits: A damaged power cable leading to the motor can cause a current leak.
• Failed Servo Amplifier: The internal components of the drive (IGBTs) may be failing, causing it to misread the current draw.
• Incorrect Parameters: If the “Current Loop” or “Thermal Protection” parameters were recently changed, the alarm might be triggering prematurely.

Motor Failure

• Internal Short: The motor windings may be contaminated with coolant or oil.
• Brake Issues: If the axis has a built-in brake (common on vertical Z-axes) and the brake is not releasing, the motor will try to “fight” the brake, leading to an immediate overload.

How to Fix Alarm 402

Follow these steps in order to isolate and resolve the issue:

Physical Inspection

Power down the machine and attempt to turn the ball screw by hand (if safe and accessible). It should move smoothly. If it feels “notchy” or requires excessive force, you have a mechanical bind. Check for chips packed into the gear tracks or damaged way-covers.

Check Lubrication

Verify that the central lubrication system is functioning. Check the oil levels and ensure the lines aren’t pinched. Manually pump the lube system and move the axis back and forth to see if the alarm clears.

Inspect Cables

Check the motor power cable (usually the thickest cable) and the feedback/encoder cable for nicks, burns, or coolant ingress. A “ground fault” in the cable will often manifest as a 402 alarm.

Test the Brake (Z-axis only)

If the alarm is on a vertical axis, ensure the 24V signal is reaching the brake to release it. If the brake remains engaged, the motor will overload within seconds of trying to move.

Swap the Servo Amplifier

If you have an identical machine or another identical axis on the same machine, you can swap the servo amplifiers. If the alarm follows the amplifier, the drive is bad. If the alarm stays on the same axis, the motor or cable is the problem.

Summary Table

Likely Cause	Quick Fix
Mechanical Jam	Clear chips, check way-wipers, and verify alignment.
Lube Failure	Fill reservoir, clear clogged lines, and grease ball screws.
Motor Brake	Check 24V supply to the brake solenoid.
Contamination	Clean motor terminals and check for coolant in the junction box.