Overview
When a fault code appears on your PLC, drive, or robot, the clock starts ticking. This guide covers the most common fault codes across major platforms with plain-language explanations and first-response actions. Bookmark this page — you'll need it at 2 AM.
Allen-Bradley CompactLogix / ControlLogix
Major Faults (Processor Stops)
| Code | Type | Meaning | First Check |
|---|---|---|---|
1 | Power-Up | Processor powered up in fault mode | Check program, download if needed |
3 | I/O | I/O module communication lost | Check I/O tree, reseat modules, check wiring |
4 | Program | Instruction execution error (e.g., divide by zero) | Check fault routine, review logic at fault point |
6 | Watchdog | Task overran its watchdog timer | Reduce scan time load or increase watchdog value |
11 | Motion | Motion group fault | Check servo drives, axis faults, encoder wiring |
Minor Faults (Processor Continues Running)
| Code | Type | Meaning | First Check |
|---|---|---|---|
2 | Watchdog | Task overlap — tried to run while already running | Optimize program or increase task period |
4 | I/O | Module not responding but not critical | Check connection, RPI settings |
7 | Storage | Memory issue | Check memory usage, remove unnecessary tags |
10 | Motion | Non-fatal motion warning | Check axis warnings in motion properties |
Recovery: Most major faults require clearing via Studio 5000 or the HMI. Type 3 (I/O) faults often self-clear once the module reconnects. Type 4 (Program) faults require fixing the code.
Allen-Bradley PowerFlex Drives (525, 527, 755)
| Code | Name | Meaning | First Check |
|---|---|---|---|
F2 | Aux Input | Auxiliary input fault triggered | Check external fault wiring to drive terminals |
F4 | UnderVoltage | DC bus voltage dropped too low | Check incoming power, loose connections |
F5 | OverVoltage | DC bus voltage exceeded limit | Motor decelerating too fast — increase decel time |
F7 | Motor Overload | Motor thermal overload | Check motor amps, ventilation, mechanical binding |
F12 | HW OverCurrent | Hardware overcurrent detected | Check motor wiring, ground faults, drive sizing |
F13 | Ground Fault | Ground fault on output | Check motor cable insulation, connections |
F29 | Analog Input Loss | 4-20mA signal lost | Check analog wiring, transmitter power |
F63 | SW OverCurrent | Software overcurrent limit hit | Check motor params, autotune drive |
F64 | Drive Overtemp | Drive heatsink too hot | Clean fans, check ventilation, ambient temp |
F100 | Parameter Default | Drive at factory defaults | Parameters need to be reconfigured |
F122 | IO Loss | Network communication lost | Check Ethernet cable, switch, PLC connection |
F12, F13, and F64 need root cause investigation before resetting — resetting repeatedly can damage the drive.
Siemens S7-1200 / S7-1500
Diagnostic Buffer Codes
| Code Range | Category | Common Causes |
|---|---|---|
16#0001-00FF | Internal | CPU hardware errors — contact Siemens support |
16#2500-25FF | I/O Access | Module removed, broken, or misconfigured |
16#3500-35FF | PROFINET | Network issues — cable, switch, or device offline |
16#4562 | Watchdog | OB1 scan time exceeded — optimize program |
16#6000-6FFF | User Program | Errors in user code (null pointers, array overruns) |
PROFINET Diagnostic LEDs
| LED Status | Meaning | First Check |
|---|---|---|
| BF solid red | Bus fault — no PROFINET partner | Check cable, switch, IP address |
| BF flashing red | Configuration error | Hardware config doesn't match physical device |
| SF solid yellow | System fault | Check diagnostic buffer in TIA Portal |
| MAINT flashing | Maintenance required | Check module diagnostics |
Recovery: Siemens provides detailed diagnostics through TIA Portal's online diagnostics view. The diagnostic buffer (accessible even offline from the front panel on S7-1500) gives the exact timestamp and cause.
FANUC Robot Controller (R-30iB / R-30iB Plus)
| Code | Name | Meaning | First Check |
|---|---|---|---|
SRVO-001 | OP E-Stop | E-stop pressed on operator panel | Release E-stop, reset |
SRVO-002 | TP E-Stop | E-stop pressed on teach pendant | Release TP E-stop, reset |
SRVO-004 | Fence Open | Safety fence circuit open | Check gate switches, safety relay |
SRVO-006 | Hand Broken | Deadman switch released | Squeeze deadman on TP |
SRVO-007 | Ext E-Stop | External E-stop input active | Check external safety chain |
SRVO-012 | Power Failure | Servo power lost unexpectedly | Check main breaker, incoming power |
SRVO-023 | Servo Alarm | Servo amplifier fault on axis | Check drive, motor, encoder cable |
SRVO-037 | IMSTP Input | Immediate stop input active | Check PLC handshaking signals |
SRVO-050 | Collision | Collision detection triggered | Check for obstruction, adjust sensitivity |
MOTN-018 | Pos Not Reached | Couldn't reach commanded position | Check reach, payload, interference |
HOST-002 | Comm Fail | Ethernet communication timeout | Check cable, PLC program, EtherNet/IP |
SYST-011 | CMOS Bad | Backup battery dead, memory lost | Replace battery, restore from backup |
Recovery: Most SRVO alarms clear with FAULT RESET after the cause is resolved. SYST-011 (CMOS Bad) is critical — you need a backup to restore.
Quick Troubleshooting Decision Tree
Fault appeared → Is production down?
├── YES → Is it a safety fault (E-stop, gate, light curtain)?
│ ├── YES → Check safety chain, reset when safe
│ └── NO → Is it a communication fault?
│ ├── YES → Check cables, switches, IP addresses
│ └── NO → Is it a drive/motor fault?
│ ├── YES → Check motor, wiring, mechanical load
│ └── NO → Read fault code details, check this guide
└── NO → Log the fault, investigate during next planned downtimePro Tips
- Screenshot your faults — Before resetting, take a photo of the fault screen including timestamp and details.
- Check the fault log, not just the active fault — The first fault in a cascade is usually the root cause. Later faults are symptoms.
- Don't just reset repeatedly — If a fault keeps coming back, something is wrong. Resetting over and over can damage equipment (especially overcurrent and ground faults on drives). If it won't clear, it's time for emergency support.
- Keep this guide accessible — Print it, bookmark it, put a copy at each panel. Fault codes are useless if you can't look them up fast.
Fault Got You Stuck?
Some faults need hands-on troubleshooting from someone who's seen them before. Our 24/7 emergency support team covers Allen-Bradley, Siemens, FANUC, and KUKA systems. For recurring issues, our PLC programming team can build better fault handling into your system.
Get Emergency Support Call (615) 854-2420