The short answer
A major fault means the controller hit a condition it could not run past: it stopped the logic, set the outputs to their faulted state and flashes the OK indicator red (MOD on a ControlLogix 5590). Studio 5000 shows the fault as a type and a code – T04:C20 is type 4, code 20 – on the Major Faults tab of Controller Properties. The type tells you where to look: 1 power-up, 3 I/O, 4 an instruction in the program, 6 a task watchdog, 7 nonvolatile memory, 11 motion.
This covers Allen-Bradley ControlLogix (1756) and CompactLogix (1769, 5069) controllers in Studio 5000 Logix Designer, the Rockwell Automation software formerly called RSLogix 5000.
Key takeaways
- Read the type and code before anyone clears anything – the Major Faults tab shows only the last three faults.
- Clear Majors restarts the logic at once if the keyswitch is in RUN: make the machine safe first.
- Don't clear faults in a loop from a fault routine; Rockwell says to clear only the specific fault you expect.
Major, minor and I/O faults
- Major fault – logic stops, outputs go to their configured faulted state, and the controller stays faulted until the fault is cleared. Controllers with a scrolling status display show
Major Fault TXX:CXX. - Minor fault – logged on the Minor Faults tab while the controller keeps running, such as a missing or low battery (
T10:C10) or a periodic task overlap (T06:C02). - I/O fault – a module connection problem, shown as a hexadecimal code such as
16#0204on the module's Connection tab and a yellow warning symbol in the I/O tree. It becomes a major fault (T03:C16) when the module is set to major-fault the controller if its connection fails in Run mode.
Where to read the fault in Studio 5000
- Go online with the controller; don't download or change modes yet.
- Open Controller Properties, Major Faults tab. Recent Faults lists the last three major faults, newest first, with type, code and description.
- Check the Minor Faults tab: a low battery or task overlap is often the warning that came first.
- For an I/O fault, open the module's Properties, Connection tab.
In logic, a GSV instruction reads a program's MajorFaultRecord: timestamp, type, code and fault-specific data.
Common major fault codes
| Type:code | What it means (our words) |
|---|---|
T01:C01 | Power was lost and restored while the controller was in Run mode. |
T01:C40 | The battery or energy storage module couldn't save the program at power-down. |
T01:C60–C62 | A nonrecoverable fault; diagnostics can be saved for Rockwell support. |
T03:C16 | A required I/O module's connection failed. |
T03:C20 | The chassis information couldn't be read; the chassis may be bad. |
T03:C23 | A required I/O connection wasn't made before the controller went to Run. |
T04:C20 | An array index is too large, or a CONTROL structure's POS or LEN is invalid. |
T04:C31 | A JSR or RET passed fewer parameters than the routine expects. |
T04:C34 | A timer has a negative preset or accumulated value. |
T04:C83 | A value is out of range, typically a BOOL array or bit index. |
T04:C990–C999 | A user-defined fault: your own program shut the controller down. |
T06:C01 | A task didn't finish within its watchdog time. |
T07:C42 | The project on the memory card is for a different firmware revision. |
First checks by fault type
- Type 1: find the power event. For
C40, replace the battery or let the ESM charge fully; if the ESM itself has failed, replace it (or the controller, where the ESM can't be removed). ForC60–C62, Rockwell's procedure is to insert a memory card and restart so the controller saves its diagnostics, then clear the fault, download the application again and call Rockwell support with the nine-character code from the display. - Type 3: is the module seated and powered, its slot right in the I/O tree, its cable or network connected? A keying mismatch shows as
16#0114or16#0116, a module that doesn't answer as16#0204. ForT03:C23, wait for the controller's I/O indicator to turn green before switching to Run. - Type 4: the code says which kind of instruction failed; check the index, preset or parameter values that logic uses. If the fault comes right after switching to Run, Rockwell says to look at the prescan.
- Type 6: look for an endless loop, a task that has grown too long or a higher-priority task starving it. Rockwell's remedies: raise the watchdog, shorten the code, change priorities or move code to another controller.
- Type 7: update the controller firmware so it matches the project stored on the card.
How to clear a major fault safely
- Record it: screenshot the Major Faults tab. Clearing from the software empties the log.
- Fix the cause, or the fault comes straight back.
- Make the machine safe with the people who run it: clearing can restart the logic.
- Clear it: select Clear Majors on the Major Faults tab – with the keyswitch in RUN, the controller runs again immediately. Or turn the keyswitch from RUN to PROG, then REM, then RUN (or back to PROG to stay stopped); that clears the fault but keeps the log.
- Check the indicators: OK should go from flashing red to solid green. On a ControlLogix 5580, steady red means something else, such as power-up diagnostics or a controller that is powered but inoperable.
Safety first: work inside a control cabinet only as a qualified person, under your site's lockout/tagout procedure, and follow Rockwell Automation's procedure for the controller. Never force outputs, jumper interlocks or bypass a safety circuit to get a machine running after a fault.
When the fault keeps coming back: what to collect
- Type, code and description of every occurrence, with date and time.
- Controller catalog number, series and firmware, and the Logix Designer version.
- What the machine was doing: starting up, changing product, powering up.
- For I/O faults, the module, its slot or address and its Connection tab code; for watchdog faults, the task's scan time against its watchdog.
- Minor faults logged before the major one, and recent program or hardware changes.
That is what Rockwell support or an integrator will ask for; our engineering team does PLC programming and changes. Replacing hardware? Read Allen-Bradley catalog numbers and series first, and for older systems the SLC 500 migration guide.
Parts that commonly fail
Controllers, communication modules, chassis, power supplies and batteries are the hardware behind many of these faults. The part numbers we have the most of in stock right now:
ControlLogix controllers
ControlLogix communication modules
ControlLogix chassis and power supplies
ControlLogix and CompactLogix I/O modules
Stock changes every day. Every listing is photographed, so check the catalog number, series or product version and firmware on the label photo before you buy.
Not listed, or need several? Send us the part number and we'll check our shelves and our sourcing network and quote it in writing.
Sources
Every code, LED state and menu path on this page was checked against Rockwell Automation's own documents, listed below, and is described here in our own words. Manuals change between editions and firmware revisions: for your exact catalog or order number, check the current edition.
- Rockwell Automation, Major, Minor, and I/O Faults Programming Manual, publication 1756-PM014O-EN-P, September 2024.
- Rockwell Automation, Logix 5000 Controller and I/O Fault Codes (spreadsheet), publication 1756-RD001F-EN-P, September 2025.
- Rockwell Automation, Studio 5000 Logix Designer online help: Clear faults, version 38.00.
- Rockwell Automation, Studio 5000 Logix Designer online help: Major Faults tab settings, version 38.00.
- Rockwell Automation, ControlLogix 5580 and GuardLogix 5580 Controllers User Manual: Controller Status Indicators, publication 1756-UM543.
- Rockwell Automation, ControlLogix 5590 Controller User Manual: Fault Messages, publication 1756-UM900.
Frequently asked questions
What does Major Fault T04:C20 mean?
Type 4 is a program fault: an instruction could not run. Code 20 means an array subscript was too large, or the POS or LEN value of a CONTROL structure was out of range. Check the index tag used by the faulting logic against the size of the array, and fix the logic before you clear the fault.
How do I clear a major fault on a ControlLogix or CompactLogix controller?
Find and fix the cause first. Then, online in Studio 5000, open Controller Properties, go to the Major Faults tab and select Clear Majors; if the keyswitch is in RUN, the controller starts running again straight away. On a controller with a keyswitch you can also turn it from RUN to PROG, then REM, then RUN; that clears the fault but keeps the fault log.
What is the difference between a major and a minor fault?
A major fault stops the logic, sends the outputs to their configured faulted state and makes the OK indicator flash red. A minor fault is only logged on the Minor Faults tab while the controller keeps running, for example a missing or low battery (type 10, code 10) or a periodic task overlap (type 6, code 2).
Why does the controller show T01:C01 after a power cut?
Type 1, code 1 means power was lost and restored while the controller was in Run mode. When the Power-Up Handler has no logic, the controller raises this fault instead of going back to Run by itself. Make the machine safe, then clear it; a restart without a person needs logic in the Power-Up Handler that clears exactly that fault.