ControlLogix Major Fault Type 6 Code 1 (Watchdog): Causes

Q: What causes Rockwell Automation / Allen-Bradley ControlLogix / CompactLogix (Logix 5000) fault T06:C01?

1. Infinite or very long loop (JMP/LBL, FOR, WHILE): Review loops in the faulted task for an exit condition that can fail to be met. 2. Task has more logic than its watchdog allows: Compare maximum scan time with the watchdog; profile which programs take the time. 3. Higher-priority tasks starve this task: Review task priorities, periods and their scan times; check for task overlaps in the minor faults.

A user task did not complete within its watchdog time. Rockwell lists three causes: a program error caused an infinite loop, the program is too complex to run as quickly as specified, or a higher-priority task is keeping this task from finishing. Corrective actions: increase the task watchdog, shorten execution time, raise the task priority, simplify higher-priority tasks, or move code to another controller.

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When it happens

In Run, when a task scan exceeds its watchdog
After a program change that added loops or logic

Likely causes and checks

1Infinite or very long loop (JMP/LBL, FOR, WHILE)

Points to it: Fault after an edit that added a loop; scan time jumps rather than creeps.
Check: Review loops in the faulted task for an exit condition that can fail to be met.

2Task has more logic than its watchdog allows

Points to it: Task scan times (Task Properties > Monitor) trend close to the watchdog before the fault.
Check: Compare maximum scan time with the watchdog; profile which programs take the time.

3Higher-priority tasks starve this task

Points to it: Periodic or event tasks with higher priority run often or long; the faulted task itself is small.
Check: Review task priorities, periods and their scan times; check for task overlaps in the minor faults.

Telling it apart

Raising the watchdog only hides an infinite loop; find out whether the task normally finishes well within it before changing the setting.
Task overlap shows as a minor fault (FaultLog MinorFaultBits bit 6) and is a warning sign that tasks are competing for time.
Type 6 faults are handled by the Controller Fault Handler, not the program fault routine.

Relevant parameters

Parameter	Name	Why it matters
Task Properties > Configuration	Task watchdog	Time the task may run before T06:C01; increase only after confirming the cause.
Task Properties > Configuration	Task priority and period	Higher-priority tasks interrupt this one; Rockwell lists priority changes as a corrective action.

Evidence to collect

Major Faults tab screenshot (names the task)
Task Properties: watchdog, priority, period and max scan time
Minor Faults tab (task overlaps)
Program export of the task’s programs
Recent online edits

Work on fuel trains, burners, pressure vessels and electrical equipment must be done by qualified personnel following the manufacturer’s documentation and local codes. Never bypass a safety device or interlock.

Rockwell Automation / Allen-Bradley ControlLogix / CompactLogix (Logix 5000) fault T06:C01: Watchdog fault: task watchdog expired