The short answer
An Allen-Bradley controller that won't go to RUN is usually held back by one of four things: the keyswitch (software can change the mode only with the switch in REM), a major fault that hasn't been cleared, a project or firmware that doesn't match the controller, or I/O that doesn't match the configuration. Read the fault first: Studio 5000's Major Faults tab on ControlLogix and CompactLogix, status word S:6 on an SLC 500 or MicroLogix. Forces don't block RUN, but they change what the machine does once it runs.
This covers Allen-Bradley ControlLogix (1756), CompactLogix (1769, 5069), SLC 500 (1747) and MicroLogix controllers from Rockwell Automation.
Key takeaways
- Look at the keyswitch before the laptop: in RUN or PROG, the software can't change the mode.
- A major fault must be found, fixed and cleared before the controller accepts a mode change.
- Program mode is not an emergency stop – Rockwell says so in plain words.
1. Check the keyswitch or mode switch
| Position | What the controller does (our words) |
|---|---|
RUN | Runs the program. Downloads, online edits and firmware updates are blocked, and the software can't change the mode. |
REM | Remote: the software chooses Remote Run, Remote Program or Remote Test. Turning the key from RUN to REM gives Remote Run, from PROG to REM Remote Program. |
PROG | Program mode: no scan, outputs at their Program-mode state; the software can't change the mode. |
ControlLogix 5560, 5570 and 5580, CompactLogix 5370 and 5380 and the SLC 5/03, 5/04 and 5/05 all have these three positions; the details of what each blocks vary by model. The SLC 5/01 and 5/02 have no keyswitch, so their mode changes over communications. The MicroLogix 1100 and 1400 have the switch on the LCD menu (PROGRAM, REMOTE, RUN); holding OK for 5 seconds at power-up forces it to RUN – only when the machine is safe to start – and ESC to PROG. An SLC can take up to 2.5 seconds to change modes after the key moves.
2. Look for a major fault
- ControlLogix and CompactLogix: open Controller Properties, Major Faults tab. Codes that block RUN directly:
T03:C23(a required I/O connection wasn't made before going to Run – wait for the I/O indicator to turn green),T08:C01(the key was turned to Run during a download – let the download finish, then clear) andT07:C41/C42(the project on the memory card is for another controller type or firmware revision). Our ControlLogix fault code guide covers the rest. - After a power cut: if the project has a Power-Up Handler with an empty routine, the controller powers up faulted with
T01:C01and waits. Don't assume it will wait – check the project before restoring power. - SLC 500: a flashing FLT LED and bit
S:1/13set. UntilS:1/13is cleared the processor won't accept a mode change. If it powers up stopped with the key in RUN and RSLogix 500 shows Remote Download in Progress withS:1/0–S:1/4all clear, Rockwell suspects a damaged keyswitch and asks you to contact them. See the SLC 500 fault code guide. - MicroLogix: a flashing FAULT LED; read
S:6. See the MicroLogix FAULT LED guide.
3. Does the project match the controller?
- Logix firmware: the controller firmware and Logix Designer must be the same major revision – firmware 38.xxx needs version 38. If they differ, the download offers to update the controller's firmware first, which needs Program or Remote Program and no recoverable major faults.
- CompactLogix 5370: when a download finds a firmware mismatch, AutoFlash can load the matching firmware over EtherNet/IP or USB.
- Logix serial number: with Match Project to Controller on, a download to a different controller (a replacement, say) stops until you confirm it.
- SLC 500: going-to-run codes
0010–0019, such as0010(processor revision not compatible with the program),0011(program file 2 missing),0013(required memory module missing) and0018(program too advanced for the operating system). - MicroLogix:
0050–0052– the controller type, base type or minimum series in the program doesn't match the hardware.
4. Does the I/O match the configuration?
- ControlLogix 5560 I/O indicator: off means no devices in the I/O configuration or no project in memory (download it); flashing green, one or more configured devices not responding; flashing red, a chassis fault.
- SLC 500:
xx52(module missing),xx53(module in a slot the program calls empty),xx54(wrong type),xx55(wrong I/O count),xx56(chassis configuration), wherexxis the slot in hex. The processor works only in slot 0 of the first chassis. - MicroLogix:
xx87means the expansion I/O in the program doesn't match what is fitted;xx71, a module the controller can't talk to.
5. Running but not doing anything: forces and logic
Forces don't stop a controller entering RUN, but they override the logic. On a ControlLogix 5560, a steady yellow FORCE indicator means forces are enabled; flashing means forces are installed but not enabled – enabling them puts every one into effect at once. On an SLC, RUN on with FORCE on can mean forced I/O is holding the machine; RUN on with nothing else lit points at the program logic or the field I/O. Power on but RUN off can also mean an SUS instruction has suspended the processor.
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, and never use Program mode or the keyswitch as an emergency stop.
If it still won't run: what to collect
- The keyswitch position, every status LED and the fault type and code.
- Controller catalog number, series and firmware; the software version and the project revision.
- What changed: a controller or module swap, a download, a firmware update, a power cut.
Our engineering team does PLC programming and changes, including projects that won't download. Battery or memory trouble? See the PLC battery guide.
Parts that commonly fail
Controllers, I/O modules and replacement keys are the hardware behind many of these checks. The part numbers we have the most of in stock right now:
ControlLogix controllers
SLC 500 processors
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, ControlLogix 5570 and 5560 Controllers User Manual, publication 1756-UM001Q-EN-P, December 2024.
- Rockwell Automation, ControlLogix 5580 and GuardLogix 5580 Controllers User Manual, publication 1756-UM543T-EN-P, December 2025.
- Rockwell Automation, CompactLogix 5380 and Compact GuardLogix 5380 Controllers User Manual, publication 5069-UM001N-EN-P, January 2026.
- Rockwell Automation, CompactLogix 5370 Controllers User Manual, publication 1769-UM021K-EN-P, May 2023.
- 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, SLC 500 Modular Hardware Style User Manual, publication 1747-UM011G-EN-P, June 2008.
- Rockwell Automation, SLC 500 Instruction Set Reference Manual, publication 1747-RM001G-EN-P, November 2008.
- Rockwell Automation, MicroLogix 1400 Programmable Controllers User Manual, publication 1766-UM001P-EN-P, March 2024.
- Rockwell Automation, MicroLogix 1200 and MicroLogix 1500 Instruction Set Reference Manual, publication 1762-RM001H-EN-P, July 2014.
Frequently asked questions
Why can't I change the controller to Run from Studio 5000?
The keyswitch decides. Logix Designer can change the mode only when the switch is in REM; in RUN or PROG the software can't switch modes. Turn the key to REM, or turn it to RUN by hand once the machine is safe to start.
Why does the SLC go straight back to fault when I switch to RUN?
The cause is still there. Read the code in S:6: going-to-run errors mean the processor's revision doesn't suit the program (0010), program file 2 is missing (0011) or a required memory module is absent (0013); I/O codes xx52 to xx56 mean the configured modules don't match the chassis.
Do forces stop a PLC from going to RUN?
No. Forces don't block the mode change, but with forces enabled the forced inputs and outputs ignore the logic, so the machine may not behave as the program says. Find them online first; enabling or adding a force takes effect at once.
What is major fault T03:C23 on a ControlLogix or CompactLogix?
At least one I/O connection set to major-fault the controller wasn't made before the controller went to Run. Rockwell's fix is to wait until the controller's I/O indicator is green before changing to Run, and to find the module that isn't connecting.