The CompactLogix controller shows a solid or flashing red FAULT light, the CPU may have stopped executing the routine, and connected I/O or an HMI reports a loss of controller.
These are the fault branches we see most often on a CompactLogix for a major fault (red FAULT LED) condition. A major fault is latched diagnostic state, and the Type/Code read from RSLogix is what splits it into program, I/O or power. Likelihood reflects field frequency; First Check is what you can do remotely before sharing logs.
| Cause | Likelihood | First Check |
|---|---|---|
| Program / controller fault (Type 01 class) — watchdog, indirect address, math overflow | High | Open the Major Faults folder in RSLogix/Studio 5000 and note the Type, Code and the routine it occurred in |
| I/O fault (Type 02 class) — module not responding, connection timeout, rack fault | Medium | Compare the I/O tree to the physical modules and read each module's own status LED |
| Configuration mismatch after a download (wrong revision or missing module) | Medium | Compare the project's controller revision and options to the hardware |
| Power interruption leaving a latched major fault | Low | Check the diagnostic history for a power-loss entry and the DC supply |
Work through these in order. Each step is something you can perform and report back on without opening the enclosure or swapping parts. Do not clear the fault before reading it — clearing wipes the clue.
In RSLogix 5000 / Studio 5000, expand the controller and open the Major Faults folder. Capture the Type, the Code and the routine name. The exact text is what splits program vs I/O vs power.
A Type 01 points to program logic; a Type 02 points to an I/O or connection issue. Note the fault description and the referenced routine so we can address the right branch.
Compare the configured modules in the I/O tree to the physical rack and each module's status LED. A missing or faulted module explains a Type 02 fault and tells us where to look.
Look for a power-loss or unexpected stop entry. A retained fault after a brownout is common and often clears on a controlled power cycle once the cause is understood.
Confirm the project's controller revision and options match the hardware. A download to the wrong revision or with a missing module logs a configuration fault that surfaces as a major fault.
A red FAULT LED on a CompactLogix is a latched major fault, not necessarily dead hardware. The Type/Code read from RSLogix is the whole story — once we know it, we can tell you whether the cause is in the program, the I/O or the power. Avoid clearing the fault before reading it; clearing wipes the clue that tells us which branch to address. Cross-reference the broader collections: Major fault / fault LED · PLC & controller diagnostics.
When to escalate: if the fault returns immediately after a clear and points to a program routine, it is a logic issue we address from the code; if it is power-related and repeats, the DC supply or a brownout upstream needs attention. We do not swap the controller to resolve a major fault — a hardware swap is only suspected when the fault pins to a physical module and we confirm that from the readings.
Almost never. A major fault is latched diagnostic state; we address the cause from the Type/Code you read. Hardware is only suspected if the fault pins to a physical module and we confirm it from the readings.
A download to a mismatched revision or with a missing module logs a configuration or I/O major fault. We compare the project to the hardware and tell you what to correct before the fault returns.
No. You clear it in RSLogix; we interpret the Type/Code first so it does not simply return. We do not take control of your controller.
Sometimes, if the fault is a retained power-loss event. But if it reappears, the underlying cause is still present and we address that from the code rather than masking it with a reset.
Send the Major Faults folder readout and the I/O tree screenshot. We split program, I/O and power from the Type/Code remotely so you avoid swapping a good controller.
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The program-timeout sibling of a major fault.
Another machinery-controller fault read from the diagnostic buffer.
A different controller-brand fault we read remotely.
Full machinery controller troubleshooting collection.