Fanuc 7115 Alarm Page

Troubleshooting the FANUC 7115 Alarm: “Illegal Command in G-Code”

If you run CNC machines long enough, you’ll eventually see the red screen of frustration. One alarm that stops operators in their tracks is the FANUC 7115 alarm.

Seeing 7115 ILLEGAL COMMAND on your FANUC control (especially on Series 0i, 16i, 18i, 21i, or 31i) can feel like a dead end. But don’t panic. In 90% of cases, it’s a simple fix—not a broken servo or a dead motherboard. fanuc 7115 alarm

Let’s break down what this alarm actually means and how to get your machine running again. Troubleshooting the FANUC 7115 Alarm: “Illegal Command in

2. Spindle Drive Malfunctions

• Feedback Failure: If the spindle encoder or motor sensor is failing, it may send erratic pulse data to the control, leading the system to believe the motor is spinning faster than it actually is.
• Loop Gain Issues: Incorrect loop gain parameters in the servo/spindle drive can cause the motor to hunt or oscillate violently, potentially triggering an overspeed detection.

Parameter 3004 (Servo Parameter)

| Bit | Function | Recommended Value | | :--- | :--- | :--- | | #5 (RCD) | Reference point return direction check | 1 (Skip check – default for most machines) | | | | 0 (Check enabled – more strict, may cause 7115) | Feedback Failure: If the spindle encoder or motor

Note: Setting 3004#5 = 1 masks the alarm but does not fix the underlying cause. It is acceptable on most standard machine tools but should be used with caution.

Step-by-Step Troubleshooting

When the alarm hits, do this in order:

1. Write down the line number shown on the alarm screen (e.g., "N0450").
2. Press SYSTEM → ALARM (or MSG button) to get the full details.
3. Go to the MDI mode and press PROG. Use the arrow keys to scroll to the offending line number.
4. Look for these red flags:
   • Letters: O, I (eye) instead of 0, 1
   • Spaces inside a word (e.g., G 00)
   • Unsupported codes like G99.9
   • Negative values where they don't belong (e.g., X-2. in a drill cycle)
5. Cross-check the questionable G-code against your machine’s manual. Just because it’s in a generic FANUC book doesn’t mean your builder turned it on.