FANUC Robot Alarm Diagnosis and Program Fault Correction

FANUC Robot Alarm Diagnosis and Program Fault Correction

Why Alarm Diagnosis Matters

A FANUC robot that faults and stops costs production time. Your job as a mechatronics technician is to find the root cause fast, fix it correctly, and return the cell to normal operation — without creating a new hazard. This lesson walks through the alarm system, I/O status tools, and ROBOGUIDE simulation so you can do exactly that.

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Safety Before You Touch Anything

Before you open any panel or reach into a cell, apply the correct hazard controls:

• Lockout/tagout (LOTO): OSHA 29 CFR 1910.147 requires you to de-energize and lock out the robot controller before any maintenance that requires defeating safeguards.
• Machine guarding: OSHA 1910.212 and ANSI/RIA R15.06 / ISO 10218 define the safeguarding requirements for robot cells — safety fences, interlocked gates, and light curtains must be functional before auto mode is restored.
• Electrical safety: Follow NFPA 70E. Verify absence of voltage with a properly rated meter before touching conductors inside the controller cabinet.
• Teach-pendant work inside the cell: When jogging the robot with the teach pendant, the deadman (enabling) switch must be held in the mid-position. Speed is limited by the teach-pendant speed override — keep it low (≤10%) when near a seeded fault until you understand the robot's state.
• Never bypass or jumper a safety gate to clear an alarm. If the gate interlock is triggering a fault, investigate why before re-enabling.

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FANUC Alarm Code Structure

Every FANUC alarm has three parts: a severity level, an alarm code number, and a title string. The four severity levels are:

| Level | Effect on robot |

|---|---|

| WARN | Warning only; cycle continues |

| PAUSE | Motion pauses; program can be restarted |

| ABORT | Program aborted; must restart |

| STOP | Emergency stop; servo power cut |

Alarm codes follow the pattern SRVO-023 or MOTN-018, where the prefix identifies the subsystem (SRVO = servo, MOTN = motion, INTP = interpreter, I/O = I/O system, etc.) and the number identifies the specific fault.

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Navigating Alarm History

On the teach pendant, press MENUS → Alarm → Alarm Log to open the Alarm History screen. The most recent fault appears at the top. For each entry you can see:

1. The alarm code and title

2. The date and time it occurred

3. The program name and line number active when the fault fired

Always read the most recent alarm first, but scroll down to see if earlier alarms contributed. A chain of alarms often tells the story: a missing I/O signal may cause a motion timeout, which then causes a program abort.

Press F5 (DETAIL) on any alarm line for an extended description and, in many cases, a suggested remedy.

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Using the I/O Status Screen

Press MENUS → I/O to open the I/O status screen. From here you can monitor:

• Digital Inputs (DI) and Outputs (DO): Each point shows its simulated value and its physical rack/slot/point assignment.
• UOP (User Operator Panel): Signals such as ENBL, FAULT RESET, and START come through here.

Distinguishing a missing signal from a program logic error:

• If the DO that should open the end-effector reads ON but the end-effector does not move, suspect a hardware or wiring problem (or an incorrect rack/slot mapping).
• If the DO reads OFF when the program should have turned it ON, open the program and check the output instruction on that line — the wrong DO number may be programmed, or a conditional branch may have skipped the instruction.
• Use I/O → Config to verify each DO is mapped to the correct rack, slot, and start point. A common error is assigning DO[1] to rack 1, slot 1, start 1 when the physical board is at rack 1, slot 2.

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Three Common Faults and How to Fix Them

1. Tool-Frame Offset Error (Position Error Alarms)

If the tool center point (TCP) Z value is entered incorrectly, the robot will place the TCP at the wrong location. Motion instructions that seemed correct during teach will now reach target positions with the wrong approach distance, causing MOTN position-deviation alarms or tool collisions. Fix: Navigate to MENUS → Setup → Frames → Tool Frame, select the active tool frame number, and correct the Z offset to the true measured value. Re-verify all taught positions before running auto cycle.

2. I/O Output Not Mapped Correctly

If a DO is assigned to the wrong rack/slot, toggling it in the program has no effect on the physical output card. Fix: MENUS → I/O → Digital → Config, find the DO number, and correct the rack, slot, and start fields to match the physical output board location.

3. Program Branch to Non-Existent Label

If a JMP/LBL instruction references a label that was deleted or never created, the interpreter fires an INTP alarm and aborts the program. Fix: Open the program in the program editor, find the JMP instruction, and either add the missing LBL[ ] line at the intended jump destination or correct the label number to match an existing label.

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Restoring Normal Operation

After making any correction:

1. Confirm all safeguards are in place per the ANSI/RIA R15.06 checklist.

2. Clear the alarm with FAULT RESET.

3. Jog the robot to a known home position.

4. Select the program, set speed override to a low value, and run one step-through cycle on the teach pendant.

5. If step-through is clean, return to AUTO mode and run one complete automatic cycle.

6. Document the alarm code, root cause, corrective action, and the screen/parameter you changed on the work-order form.