Major Fault Type 1 Allen-Bradley — CPU in STOP
At a glance
- Severity
- FAULT STOP
- Domain
- PLC
- Reference standard
- IEC 61131-3
- Product / context
- Power-up fault
What it means
The PLC CPU is in STOP state: the RUN LED is off and the STOP LED is solid red. The user program is not executing, all outputs are in the safe state (default to 0). No scan cycle is being executed.
Probable causes
- Programming error causing a runtime exception (null pointer access, division by zero, stack overflow).
- Physical mode selector on the CPU set to STOP instead of RUN.
- Memory card missing, corrupted, or incompatible with the firmware version.
- Failed or interrupted firmware update that left the CPU in an inconsistent state.
Where to start
Check the physical mode selector (RUN/STOP/MRES) on the CPU. Is the selector in the RUN position?
Full guided diagnosis in the app
The diagnostic tree for this fault has 7 guided steps with measurements, checks and resolutions. You'll find it in ElectraIQ, one of the five IndexOS apps: free, works offline.
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The same fault on other brands
More Allen-Bradley codes
- 1756-IB16 — Digital input module — input not reading (stuck at 0)
- 1756-IF8 — Analog input module — full-scale reading or error
- 1756-PA75 — Unstable PLC power supply — cyclic restarts
- Major Fault Type 3 — CPU ↔ I/O module communication error (backplane bus)
- Minor Fault Type 10 — Memory error / backup battery depleted
- Safety Task Fault Type 14 — Safety module F-CPU/F-DI/F-DO in passivation