The Most Common CPU Faults in Industrial Machines and How to Fix Them

In the world of industrial maintenance, the CPU (Central Processing Unit) is the brain of the machine. It receives signals from sensors, processes them based on a programmed logic, and controls the execution of operations. However, like any electronic component, CPUs can experience faults that lead to machine downtime and production stoppages. In this article, we will explore the most common faults affecting CPUs in industrial machines, along with practical troubleshooting and repair steps.

1. What is the CPU in Industrial Machines?

It is a smart electronic unit (typically a PLC – Programmable Logic Controller) that reads input signals from sensors and switches, processes them according to a logic program, and then sends output commands to actuators (such as valves, motors, or indicators).

2. Most Common CPU Faults

A. Power Supply Failure

Symptoms: POWER LED is off, the machine does not operate.

Causes: Blown fuse, disconnected cable, or faulty power supply.

Fix:

Measure the voltage at the CPU input using a multimeter.

Check for the correct supply (24VDC or 220VAC depending on the model).

Replace the power supply or fuse if needed.

B. CPU in STOP or ERROR Mode

Symptoms: RUN LED is off, FAULT or ERROR LED is flashing.

Causes:

Fault in the program.

Faulty I/O module.

Lost program due to battery issue or misload.

Fix:

Connect a PC to the CPU using the appropriate software (e.g., TIA Portal, SoMachine).

Read the diagnostic buffer for error messages.

Reload the original program if necessary.

Check and secure all external modules.

C. I/O Module Failure

Symptoms: Some inputs/outputs don’t work, FAULT LED on a module.

Fix:

Check wiring and connections.

Swap with a working module to isolate the problem.

Replace the damaged module with one of the same type.

D. HMI Screen Not Communicating with CPU

Symptoms: HMI displays “Connection Error” or “PLC Not Found”.

Causes: Damaged communication cable, IP mismatch, or incorrect protocol settings.

Fix:

Check the connection cable (Ethernet or RS485).

Verify communication settings in HMI software.

Restart both the HMI and the CPU.

3. Practical Example: Siemens S7-1200

Symptoms:

RUN LED is off

ERROR LED is blinking

Machine does not respond

Fix Steps:

1. Ensure 24VDC power is present.

2. Open TIA Portal and go online with the CPU.

3. Go to: Online > Diagnostics > Diagnostic Buffer.

4. Check the cause (e.g., faulty I/O module, missing program).

5. Resolve the issue and switch the CPU to RUN mode.

4. Tips to Prevent CPU Faults

Use a stable and clean power supply (UPS recommended).

Keep electrical panels clean and free of dust and moisture.

Always maintain a backup of every PLC program.

Monitor operating temperatures and ensure proper ventilation.

Knowing how to troubleshoot and repair CPU faults efficiently is a core skill for every industrial maintenance technician. With careful observation and the right tools like a multimeter and diagnostic software you can quickly fix most CPU-related issues and keep your machines running smoothly.