What causes fault code AB-PLC-MAJOR04 (Allen-Bradley PLC Major Fault â€” Program Execution (Type 04))?

Watchdog timer expired â€” scan time exceeded task watchdog setting due to complex logic or infinite loop. Math overflow or divide-by-zero in ladder logic or structured text. Array subscript out of range â€” program tried to access array element beyond defined size. Task overlap â€” periodic task frequency too high for available scan time. Corrupted program â€” memory error or incomplete download

How do I fix fault code AB-PLC-MAJOR04?

Connect RSLogix 5000/Studio 5000 â€” read Major Fault Code and Sub-Code from Controller Properties. Type 04 Code 16: watchdog â€” increase watchdog time in Task Properties, or optimize logic to reduce scan time. Type 04 Code 20: array index â€” find the instruction referencing the out-of-range subscript, add bounds checking. Type 04 Code 04: math overflow â€” add error checking before DIV/MOD instructions, check for zero divisor. Type 04 Code 31: task overlap â€” reduce periodic task frequency or move non-critical logic to lower priority task. Download a clean backup of the program if corruption is suspected â€” verify checksum matches known-good copy. Clear major fault and return to RUN mode â€” monitor scan time in Task Properties to confirm normal operation

Allen-Bradley PLC Major Fault â€” Program Execution (Type 04)

AB-PLC-MAJOR04

ControlLogix or CompactLogix processor encountered a program execution error. Watchdog timer expired, instruction fault, or task overlap detected. Controller went to fault mode.

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Probable Causes (ranked)

Watchdog timer expired â€” scan time exceeded task watchdog setting due to complex logic or infinite loop
Math overflow or divide-by-zero in ladder logic or structured text
Array subscript out of range â€” program tried to access array element beyond defined size
Task overlap â€” periodic task frequency too high for available scan time
Corrupted program â€” memory error or incomplete download

Fix Steps

1 Connect RSLogix 5000/Studio 5000 â€” read Major Fault Code and Sub-Code from Controller Properties
2 Type 04 Code 16: watchdog â€” increase watchdog time in Task Properties, or optimize logic to reduce scan time
3 Type 04 Code 20: array index â€” find the instruction referencing the out-of-range subscript, add bounds checking
4 Type 04 Code 04: math overflow â€” add error checking before DIV/MOD instructions, check for zero divisor
5 Type 04 Code 31: task overlap â€” reduce periodic task frequency or move non-critical logic to lower priority task
6 Download a clean backup of the program if corruption is suspected â€” verify checksum matches known-good copy
7 Clear major fault and return to RUN mode â€” monitor scan time in Task Properties to confirm normal operation

Field Trick

The fault sub-code tells you exactly what happened. Note the exact Type and Code numbers before clearing â€” they disappear after reset. Screenshot the Major Faults tab in Controller Properties.

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