PLC Error Code Lookup — Free Industrial PLC Fault Diagnosis

Q: How do I look up a PLC error code?

To look up a PLC error code: 1) Note the exact error code from the PLC's HMI display, programming software, or fault log. 2) Enter the code into RepairXpert's free diagnostic tool. 3) Get ranked root causes, step-by-step fix procedures, and replacement parts recommendations. RepairXpert covers 313+ fault codes across Allen-Bradley, general industrial, VFD, conveyor, packaging, AS/RS, and palletizer equipment.

Q: What are the main types of PLC faults?

PLC faults fall into several categories: 1) Major Faults — halt the controller entirely (power-up errors, watchdog timeouts, memory corruption). 2) Minor Faults — log warnings but the PLC keeps running (battery low, communication warnings). 3) I/O Faults — modules can't communicate with the backplane. 4) Communication Faults — network timeouts, CIP errors, Modbus failures. 5) Program Faults — math errors, array overruns, instruction exceptions. 6) Motion Faults — servo axis errors, position faults, drive faults.

Q: What does 'PLC Major Fault Type 4 Code 16' mean?

In Allen-Bradley ControlLogix, Major Fault Type 4, Code 16 is a Watchdog fault — a task took longer to execute than its configured watchdog time. This usually means: 1) A program loop is running too many iterations. 2) A task has too much code for its scan rate. 3) An instruction is waiting on a resource (message instruction timeout). Fix: increase the watchdog timer, optimize the routine, or split the task into smaller segments.

Q: How do I troubleshoot PLC communication faults?

PLC communication fault troubleshooting: 1) Check physical layer — cables, connectors, switch ports, fiber links. Look for link/activity LEDs. 2) Verify IP addresses — no duplicates, correct subnet mask, gateway. 3) Check network load — use managed switch port statistics to look for CRC errors, collisions, or excessive broadcast traffic. 4) Verify timeout settings — RPI (Requested Packet Interval) for EtherNet/IP, station timeout for Modbus. 5) Use the PLC's built-in diagnostics — message instruction error codes, connection status tags.

Q: Is RepairXpert a free PLC error code lookup tool?

Yes. RepairXpert's free tier includes instant fault code lookups, photo-based diagnostics (snap a photo of the HMI or fault display), and parts search with real-time pricing. No login or signup required. Pro ($19/mo) adds unlimited queries and diagnostic history. Enterprise ($49/mo) adds custom fault codes, API access, and MCP server integration for AI assistants.

41 PLC fault codes with AI-powered root cause analysis. Covers Allen-Bradley ControlLogix, CompactLogix, GuardLogix, and general industrial controllers. Enter any error code for instant diagnosis.

41 PLC Codes

313 Total DB

7 Equipment Types

8 MCP Tools

PLC Fault Types — Quick Reference

Understanding PLC fault categories helps you triage faster. Here's a breakdown of the major fault types you'll encounter in industrial automation:

Fault Type	Severity	Controller Behavior	Common Causes
Major Fault	Critical	Controller halts, outputs go to fault state	Watchdog timeout, memory corruption, power-up without project
Minor Fault	Warning	Controller keeps running, logs warning	Battery low, serial port error, redundancy warning
I/O Fault	High	Affected module goes offline	Module removed, backplane error, RPI timeout
Communication	High	Remote I/O or peer controller lost	Network cable, IP conflict, switch failure, RPI too fast
Program Fault	Major	Controller faults on instruction error	Divide by zero, array overrun, invalid tag reference
Motion Fault	High	Axis faulted, motion stops	Position error, overcurrent, drive fault, encoder failure

Allen-Bradley PLC Error Codes

Allen-Bradley ControlLogix and CompactLogix controllers use a Type/Code system for faults. The fault type identifies the category, and the code gives the specific error. Common examples:

Type 1, Code 1 — Power-Up: Controller powered up or was restored without a user project. Download the project from Studio 5000.
Type 4, Code 16 — Watchdog: Task exceeded its watchdog time. Optimize the routine or increase the watchdog timer.
Type 4, Code 82 — No Connections: No I/O connections established. Check module configuration and network path.
Type 6, Code 1 — Math Overflow: Instruction result exceeded the data type range. Add bounds checking to your logic.
Type 11, Code 1 — Motion Axis Fault: A servo axis reported an error. Check the Kinetix drive fault log for details.

PLC Communication Fault Troubleshooting

Communication faults are the second most common PLC issue after I/O faults. Follow this systematic approach:

Physical layer: Check cables, connectors, switch port LEDs. Look for link and activity indicators. Replace suspect cables with known-good ones.
Network layer: Verify IP addresses (no duplicates), subnet masks, and gateways. Use ping to test reachability.
Application layer: Check RPI settings (EtherNet/IP), station timeouts (Modbus), and connection parameters. Too-fast RPIs on congested networks cause intermittent faults.
Switch diagnostics: On managed switches, check port statistics for CRC errors, collisions, and packet drops. Replace failing switch ports.
PLC diagnostics: Use MSG instruction error codes, connection status tags, and the controller's diagnostic counters to identify the failing connection.

PLC Program Faults — Prevention Tips

Program faults halt the controller when an instruction encounters an unrecoverable error. Prevent them with these practices:

Array bounds checking: Always verify array indices before using them. A common cause of Type 4 faults is indexing beyond array limits.
Division guards: Check for zero before any DIV, MOD, or CPT instruction that includes division.
Message timeouts: Set appropriate timeouts on MSG instructions. A hung message can trigger a watchdog fault.
Task scheduling: Don't put heavy logic in a fast periodic task. Use event tasks or slower periodic tasks for non-time-critical code.

All PLC Error Codes in Database

Allen-Bradley (30)

AB-1756-AIFLT ControlLogix 1756-IF8 / IF16 Analog Input Over-Range medium AB-1756-ENET ControlLogix 1756-EN2T Connection Timeout — I/O Module high AB-1756-IO Allen-Bradley ControlLogix I/O Module Fault (1756 Series) high AB-1756-MEM ControlLogix Memory Full — Cannot Download medium AB-1756-MOTION1 ControlLogix Servo Axis Fault — Motion Group Error high AB-1756-PS ControlLogix 1756 Power Supply Fault critical AB-1756-REDUND ControlLogix Redundancy Switchover Fault critical AB-1756-SAFETY ControlLogix 1756-L7SP Safety Partner Fault critical AB-1756-TASK ControlLogix Task Watchdog Timeout high AB-1769-FAULT CompactLogix 1769 I/O Module Fault high AB-CLGX-FW ControlLogix Firmware Flash Required After Replacement medium AB-COMPACT-ENET CompactLogix Built-in EtherNet/IP Port Fault high AB-EN2T-FAULT Allen-Bradley EtherNet/IP Adapter Fault (1756-EN2T / 1756-EN2TR) critical AB-F001 PLC Major Recoverable Fault high AB-F002 PLC I/O Module Fault high AB-F004 PLC Battery Low low AB-F007 PLC Redundancy Switchover critical AB-F009 CompactLogix Nonvolatile Memory Fail critical AB-F014 PLC Task Watchdog Expired high AB-F018 ControlLogix EtherNet/IP Module CIP Error high AB-F019 GuardLogix Safety Signature Mismatch critical AB-F022 CompactLogix USB Port Fault low AB-GUARD-SIL Allen-Bradley GuardLogix Safety Fault â€” Safety Task Fault critical AB-MICRO800 Allen-Bradley Micro800 PLC Fault (Micro820/850) high AB-PLC-MAJOR01 Allen-Bradley PLC Major Fault â€” Power Loss (Type 01) critical AB-PLC-MAJOR03 Allen-Bradley PLC Major Fault â€” I/O Module Not Responding (Type 03) critical AB-PLC-MAJOR04 Allen-Bradley PLC Major Fault â€” Program Execution (Type 04) critical AB-PLC-MAJOR06 Allen-Bradley PLC Major Fault â€” Motion Axis Fault (Type 06) critical AB-PLC-MINOR10 Allen-Bradley PLC Minor Fault â€” Battery Low (Type 10) low AB-PLC-MINOR20 Allen-Bradley PLC Minor Fault â€” EtherNet/IP Connection Loss (Type 20) high

Asrs (1)

ASRS-WCS1 AS/RS WCS Communication Loss — Crane Offline high

General (8)

60001 PLC Communication Fault â€” Network Timeout high 60002 PLC Communication Fault â€” Serial Link Down high 60003 PLC Battery Low low 60004 Communication Timeout â€” EtherNet/IP high 60006 Communication Timeout â€” Profinet high GEN-HMI1 HMI Communication Loss medium GEN-PLC1 PLC Power Supply Overload high SAF-004 Safety PLC Firmware Mismatch critical

Vfd (2)

VFD-CL1 VFD Communication Loss high VFD-FW1 VFD Firmware / Software Fault high

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PLC Error Code FAQ

How do I look up a PLC error code?

Enter the exact error code from your PLC's HMI display or programming software into RepairXpert's free diagnostic tool. Get ranked root causes, fix procedures, and parts recommendations instantly. Covers 313+ fault codes across all major equipment types.

What are the main types of PLC faults?

Major Faults (halt the controller), Minor Faults (warnings only), I/O Faults (module communication), Communication Faults (network errors), Program Faults (instruction exceptions), and Motion Faults (servo axis errors).

What does PLC Major Fault Type 4 Code 16 mean?

Type 4, Code 16 is a Watchdog fault in Allen-Bradley ControlLogix. A task took longer than its configured watchdog time. Optimize the routine, reduce iterations, or increase the watchdog timer in Task Properties.

How do I troubleshoot PLC communication faults?

Systematic approach: check physical layer (cables, LEDs), verify network settings (IP, subnet), review timeout parameters (RPI, station timeout), check switch port statistics, and use PLC diagnostic counters and MSG error codes.

Is RepairXpert a free PLC error code lookup tool?

Yes. The free tier includes instant lookups, photo-based diagnostics, and parts search. No login required. Pro ($19/mo) adds unlimited queries and history. Enterprise ($49/mo) adds custom codes, API access, and MCP integration.

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