ControlLogix Major Fault Type 4 Code 20: Causes and Fix

Q: What causes Rockwell Automation / Allen-Bradley ControlLogix / CompactLogix (Logix 5000) fault T04:C20?

1. Indirect index computed or incremented past the array bounds: Read the index tag value at the fault and compare it with the array dimension; add a limit check before the access. 2. CONTROL structure .LEN larger than the array, or .POS out of range: Compare .LEN and .POS with the size of the array the instruction addresses. 3. Index set from outside the program (HMI, recipe, message) without validation: Trace where the index tag is written and limit-check it at the point of entry.

An array subscript was too large for the array, or the .POS or .LEN value of a CONTROL structure (used by FIFO, file and sequencer instructions) was invalid. Rockwell’s corrective action: adjust the value to be within the valid range and do not exceed the array size or its defined dimensions.

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Upload a keypad photo, the fault history or a parameter export from your Rockwell Automation / Allen-Bradley ControlLogix / CompactLogix (Logix 5000). InduQT compares the evidence and suggests the next useful checks.

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When it happens

In Run, when an indirect index reaches a value outside the array
At the change to Run (prescan), on controllers before revision 13

Likely causes and checks

1Indirect index computed or incremented past the array bounds

Points to it: Fault in logic that uses a tag as a subscript, e.g. Array[Index]; index value at or above the array length, or negative.
Check: Read the index tag value at the fault and compare it with the array dimension; add a limit check before the access.

2CONTROL structure .LEN larger than the array, or .POS out of range

Points to it: Fault on FFL/FFU, COP-like file or sequencer instructions using a CONTROL tag.
Check: Compare .LEN and .POS with the size of the array the instruction addresses.

3Index set from outside the program (HMI, recipe, message) without validation

Points to it: Fault follows an operator or recipe entry.
Check: Trace where the index tag is written and limit-check it at the point of entry.

Telling it apart

T04:C21 is the separate code for CONTROL .LEN or .POS less than 0; T04:C83 is a value out of range with Boolean arrays and bit-level addressing.
From revision 13.0 the controller automatically clears array-subscript faults during prescan (an internal handler clears Type 4 Codes 20 and 83 in prescan only); a fault at the change to Run on revision 11.x or earlier points to prescan.
If a fault routine clears the fault, the faulted instruction does not run and execution continues with the next one; data the instruction should have written is not updated.

Evidence to collect

Major Faults tab screenshot (program, routine and rung information)
Value of the index tag and array dimension at the fault
Program export (L5X) of the routine
HMI or recipe entries just before the fault
Controller firmware revision

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Rockwell Automation / Allen-Bradley ControlLogix / CompactLogix (Logix 5000) fault T04:C20: Program fault: array subscript too large, or CONTROL POS/LEN invalid