PLC Error Codes List: Complete Troubleshooting Guide for Engineers
Programmable Logic Controllers (PLCs) are reliable, but even the best systems encounter faults. When something goes wrong, PLCs generate error codes that help you quickly identify and resolve the issue.
This guide provides an easy-to-understand PLC error codes list, common causes, and troubleshooting tips for brands like Siemens, Allen-Bradley, Mitsubishi, and Omron.
Why PLC Error Codes Matter
PLC faults don’t just stop machines — they can cost hours of downtime, damage equipment, or delay production. Understanding error codes allows you to:
- Diagnose problems faster
- Prevent repeated failures
- Reduce downtime
- Maintain safe operation
- Improve troubleshooting efficiency
Common PLC Error Categories
Although each manufacturer uses different codes, most PLC errors fall into these major groups:
| Error Category | Meaning | Common Causes |
|---|---|---|
| CPU Errors | Processor fault | Firmware issues, memory corruption |
| I/O Errors | Input/output modules not responding | Loose wiring, failed modules |
| Communication Errors | Network or protocol failures | Bad cables, misconfigured IP settings |
| Memory Errors | Memory overload or corruption | Full memory, faulty memory card |
| Power Supply Errors | Low or unstable voltage | Faulty PSU, grounding issues |
| Program Errors | Invalid logic or inconsistency | Incorrect instructions, scan-time issues |
| Safety Errors | Safety circuit interruption | E-Stop active, light curtain triggered |
PLC Error Codes List by Brand
1. Siemens PLC Error Codes (S7-1200 / S7-1500)
Common Siemens Faults
| Error Code | Description | Fix |
|---|---|---|
| SF (System Fault) | Hardware or configuration error | Check diagnostics buffer |
| BF (Bus Fault) | Communication bus failure | Check PROFINET/PROFIBUS wiring |
| OB121/OB122 | Programming error (I/O access fault) | Validate addresses |
| STOP Mode 16#0040 | Hardware failure | Replace CPU if persistent |
| I/O Access Error 16#0003 | Module not reachable | Inspect module connections |
2. Allen-Bradley (Rockwell) PLC Error Codes (ControlLogix / CompactLogix)
Frequent Rockwell Error Codes
| Error Code | Meaning | Recommended Action |
|---|---|---|
| Major Fault 1/xx | Controller fault | Review Fault Handler |
| Major Fault 6/xx | I/O module failure | Re-seat or replace module |
| Minor Fault 3/xx | Memory or task error | Optimize program/tasks |
| Code 16#0204 | Connection timeout | Check Ethernet/IP connections |
| Code 16#0120 | PLC overload or scan error | Reduce cyclic load |
3. Mitsubishi PLC Error Codes (FX / Q Series)
| Error Code | Description | Fix |
|---|---|---|
| 4100 | CPU hardware error | Cycle power / replace unit |
| 6601 | I/O module fault | Check module wiring |
| 6402 | Program memory error | Re-upload program |
| C200 | Communication timeout | Inspect RS-485/Ethernet cables |
| E100 | PLC in error stop | Clear error in GX Works |
4. Omron PLC Error Codes (CP1 / CJ2 / NX Series)
| Code | Meaning | Solution |
|---|---|---|
| 0x80xx | CPU errors | Review error log |
| 0x82xx | I/O bus error | Check expansion modules |
| 0x83xx | Memory corruption | Reset & reload program |
| 0x8400 | Battery low | Replace battery |
| FALS | Fatal system error | Power cycle or replace CPU |
How to Troubleshoot PLC Error Codes
1. Check Diagnostic Tools
Most PLCs include built-in tools:
- Siemens → Diagnostics Buffer
- Rockwell → Controller Fault Handler
- Mitsubishi → GX Works Error List
- Omron → CX-Programmer/NX-Diagnostics
2. Inspect Physical Hardware
- Loose connectors
- Damaged cables
- Burnt modules
- Overheated cabinets
3. Review Program Logic
Scan for:
- Invalid addresses
- Forced values
- Watchdog/scan-time issues
4. Check Power & Grounding
Unstable voltage often triggers random faults.
5. Restore from Backup
If memory is corrupted, download the last working program.
Preventing Future PLC Errors
Use these best practices:
✔ Regularly back up PLC programs
✔ Maintain proper cabinet cooling
✔ Label and organize wiring
✔ Keep firmware updated
✔ Perform periodic network health checks
✔ Replace aging power supplies before failure
Conclusion
PLC error codes don’t need to be confusing. With a clear understanding of fault categories and common error lists from major PLC brands, you can significantly reduce downtime and troubleshoot issues faster.