FANUC Robot A05B-2680-C422 Emergency Stop Unit Fault Repair: Emergency Stop Cannot Reset and Servo Cannot Start

The FANUC robot safety system needs to simultaneously monitor the control cabinet emergency stop, teach pendant emergency stop, external emergency stop, safety doors, and other safety devices. After an abnormality in the A05B-2680-C422 emergency stop unit or related circuits, even if all emergency stop buttons on-site have been released, the controller may still maintain an emergency stop state, preventing the servo from starting.

On-site diagnostic conclusion: An emergency stop that cannot be reset is not necessarily due to damage to the A05B-2680-C422 unit itself. Before repair, first check the emergency stop buttons, dual-channel inputs, 24V control power supply, peripheral safety circuits, and connectors. Confirm that external conditions are normal before testing or replacing the emergency stop unit.

1. Main Function of A05B-2680-C422
The A05B-2680-C422 is used to process emergency stop and safety-related signals in the robot control system. After an emergency stop button, safety door, or external device is triggered, the unit transmits the safety status to the controller and prevents servo startup. Specific interfaces and applicable controllers should be confirmed based on the physical nameplate, control cabinet configuration, and electrical drawings.

2. Common Fault Manifestations
Common phenomena include all emergency stop buttons being released but the system still displaying an emergency stop, the servo turning on and immediately disconnecting, emergency stop alarms that cannot be reset, the robot stopping intermittently during operation, and the need to repeatedly operate the emergency stop button to recover. Some faults only appear after the control cabinet temperature rises or equipment vibration occurs, easily misdiagnosed as software issues.

3. First, Record Complete Alarm Information
Before inspection, record the complete alarm content displayed on the teach pendant, the time of occurrence, the current operating mode, and the robot status. Simultaneously confirm whether the fault occurs at startup, during servo activation, or during automatic operation. Different fault stages correspond to different inspection scopes; do not just record the words "emergency stop alarm."

4. Check All Emergency Stop Buttons
Check the emergency stop buttons on the control cabinet operation panel, teach pendant, and peripheral workstations one by one. Confirm each button is fully rotated and released, with no mechanical jamming. Normal button appearance does not mean internal contacts are normal; also check if the dual-channel contacts can switch synchronously based on the electrical drawings.

5. Distinguish Between Internal and External Emergency Stops
If the control cabinet and teach pendant emergency stop statuses are normal, but the system still cannot reset, continue checking the interlock signals between the external emergency stop, safety doors, safety PLC, peripheral equipment, and the robot. Use the controller's safety status screen to determine which signal segment is not satisfied, avoiding direct replacement of the emergency stop unit.

6. Check the 24V Control Power Supply
The emergency stop unit and related safety inputs typically require a stable control power supply. When checking the power supply, observe the voltage performance separately under no-load, emergency stop released, and servo startup conditions. If the voltage drops significantly at the moment of servo startup, check the control power supply, line connections, fuse protection, and whether there is a short circuit or overload in the external load.

7. Check Dual-Channel Safety Signals
Safety circuits typically use a dual-channel structure, requiring both signals to change synchronously according to specified states. If one channel conducts while the other opens, or if the switching timing and states of the two channels are inconsistent, the controller may judge the safety circuit as abnormal and refuse to reset. During troubleshooting, confirm both signals separately according to the drawings; do not connect the two channels in parallel.

8. Check Connectors and Cables
Check if the connectors related to the A05B-2680-C422 are locked and if terminals have issues like retracted pins, bent pins, oxidation, liquid ingress, or burning. Also, check the safety lines in the cables from the emergency stop buttons to the control cabinet, from the safety PLC to the robot, and in the teach pendant cable. Confirm there is no crushing, stretching, or internal wire breakage.

9. Check Safety Relays and Contacts
If external input signals are normal but the emergency stop status cannot be transmitted or reset, check the safety relays, contactors, and auxiliary contacts. Contact burning, sticking, or poor rebound may cause the two channel states to be out of sync. For faults that appear only after running for a period, also observe the temperature rise of the relays and connection terminals.

10. Check the Emergency Stop Unit Itself
After confirming the power supply, emergency stop buttons, peripheral safety signals, and wiring are normal, then check the A05B-2680-C422 unit itself. Focus on observing whether the circuit board has signs of burning, corrosion, component cracking, relay discoloration, or connector cold solder joints. Water or oil mist contamination may form leakage paths on the board surface, causing the fault to manifest as intermittent alarms.

11. Why Does the Alarm Reappear Immediately After Reset?
If the emergency stop can be briefly reset but quickly reappears, it may be related to button contact bounce, intermittent cable breaks, 24V power supply drop, unstable safety relay contact, or faulty solder joints inside the unit. In this case, record the robot posture, cabinet temperature, and peripheral equipment actions when the alarm occurs, and look for conditions that can stably reproduce the fault.

12. How to Confirm the Model Before Replacement
Before replacing the emergency stop unit, verify the complete part number, hardware version, connector type, and control cabinet configuration. Modules with similar appearances may not be directly interchangeable. Before installing another version, also confirm the interface definitions and compatible control systems. Before disassembly, photograph all plug positions and make markings to avoid incorrect reconnection during reassembly.

13. First Power-On After Replacement
After installation, first check the plugs, terminals, and ground connections, then start the control system. During the first power-on, do not immediately run production programs. First observe the emergency stop status, safety inputs, and module indicator status, confirming there are no new power, communication, or safety circuit alarms.

14. Safety Function Verification
After repair is complete, press the control cabinet emergency stop, teach pendant emergency stop, and external emergency stop separately, confirming each button can stop the robot and generate the corresponding status. Then test safety doors, light curtains, and peripheral safety devices, checking if the trigger, stop, release, reset, and restart processes are normal.

15. Robot Recommissioning Test
After all safety functions are confirmed normal, test each axis movement in manual low-speed mode, then run a complete program at low speed with no load. After confirming the robot's trajectory is normal when passing near fixtures and workpieces, gradually increase speed and conduct production tests with load, while observing if the emergency stop status becomes abnormal again.

16. Summary
When an abnormality occurs in the FANUC robot A05B-2680-C422 emergency stop unit, troubleshoot in the order of emergency stop buttons, external safety circuits, dual-channel signals, 24V control power supply, connectors, and the module itself. Only after external inputs and wiring status are confirmed normal can you accurately determine if the emergency stop unit is damaged. After repair, all emergency stop and safety devices must be tested item by item before recommissioning the robot for operation.

Repair Consultation
If you need to handle FANUC robot A05B-2680-C422 emergency stop unit faults, emergency stop cannot reset, or servo cannot start, please provide the robot body model, controller model, complete alarm screen photos, emergency stop unit photos, and control cabinet wiring photos.
Industrial Robot Repair Hotline: +86 18122009539
Contact Repair Engineer: +86 18122009539