A Programmable Logic Controller rarely fails without warning. In most cases, there are clear signs that something is wrong weeks or even months before a complete failure. Recognizing these early warning signs can mean the difference between a planned repair and an emergency shutdown that costs your operation thousands per hour.
Here are the seven most common warning signs we see at Flexa Systems — and what to do about each one.
1. Intermittent Communication Failures
Your PLC communicates constantly — with HMIs, SCADA systems, other PLCs, and I/O modules over networks like Ethernet/IP, Profinet, Modbus, or DeviceNet. When these communications become unreliable, it's a red flag.
What you'll notice:
- HMI screens go blank or show "communication lost" intermittently
- SCADA alarms for communication timeouts
- I/O modules dropping off the network briefly
- Data values freezing or showing stale readings
What it usually means: Failing communication ports, degrading network interface hardware, or loose backplane connections. While it could be a cable or network issue, if you've ruled those out, the PLC processor or communication module likely needs attention.
2. Unexpected Program Changes or Memory Errors
If your PLC reports CRC errors, memory faults, or your program seems to have changed without anyone making modifications, the processor's memory hardware may be degrading.
What you'll notice:
- Program checksum errors
- Battery backup warnings (even after battery replacement)
- Random bit changes in data tables
- Program going to fault mode after power cycle
What it usually means: EEPROM or flash memory degradation, failing RAM, or battery backup circuit issues. This is serious because it can lead to unpredictable machine behavior. Back up your program immediately if you see these signs.
3. Unexplained I/O Behavior
When outputs turn on or off unexpectedly, or inputs don't register correctly, and you've verified the field wiring and devices are fine, the problem may be inside the PLC.
What you'll notice:
- Outputs energizing when they shouldn't be
- Inputs not registering even though voltage is present at the terminal
- Analog values drifting or reading incorrectly
- LED indicators on I/O modules flickering or not matching actual state
What it usually means: Failed I/O point (individual channel burned out), degrading optocoupler circuits, backplane communication issues, or power supply problems affecting I/O modules.
4. Frequent Battery Warnings
Most PLCs have a lithium battery that maintains program memory and real-time clock when power is off. A low battery warning is normal and expected every 3-5 years. But if you're getting battery warnings shortly after replacement, something else is wrong.
What you'll notice:
- Low battery indicator returns within weeks of replacement
- Date/time resets after power cycle despite new battery
- Program lost after brief power interruption
What it usually means: The battery charging circuit or voltage regulation on the processor board is failing. The battery itself may be fine — the PLC can't maintain it properly. This puts your program at risk every time power cycles.
5. Overheating and Thermal Issues
PLCs are designed to operate in industrial environments, but they have thermal limits. When a PLC starts running hotter than normal, internal components are stressed and failure is approaching.
What you'll notice:
- PLC enclosure significantly warmer than usual
- Thermal fault indicators on the processor
- Random faults that clear after the PLC cools down
- Reduced performance or slower scan times during hot periods
What it usually means: Internal power supply degradation (generating more heat), failing cooling fans, or accumulated dust restricting airflow. While cleaning and improving ventilation helps, if the power supply is the heat source, it needs repair or replacement before it takes out the processor.
6. Power Supply Instability
The PLC power supply converts incoming AC or DC power to the regulated voltages the processor and I/O modules need. When it starts failing, everything becomes unreliable.
What you'll notice:
- Power OK LED flickering
- PLC rebooting randomly
- Multiple modules showing faults simultaneously
- Audible buzzing from the power supply module
- Voltage readings outside specification when measured at backplane
What it usually means: Capacitors in the power supply are degrading (this is the most common failure mode in aging PLCs). Capacitors have a finite life, especially in warm environments, and their failure is progressive — performance degrades over time before complete failure.
7. Increasing Scan Time
Scan time is how long the PLC takes to execute one complete cycle of your program. It should be consistent. If it's increasing without program changes, hardware performance is degrading.
What you'll notice:
- Scan time trending upward over weeks or months
- Machine timing becoming less precise
- Motion control or high-speed counting becoming unreliable
- Watchdog timer faults
What it usually means: Processor performance degradation, memory access issues, or backplane communication slowdowns. This is often an early indicator of a processor approaching end of life.
What to Do If You See These Signs
Immediate steps:
- Back up your PLC program NOW. If you don't have a current backup, this is the most important thing you can do. If the PLC fails completely without a backup, reprogramming can cost $5,000-$50,000+.
- Document the symptoms — when they occur, how often, and any patterns (time of day, temperature, specific operations).
- Check the simple things — loose connections, dirty fans, power quality, cable integrity.
Plan for repair:
Unlike a complete failure that forces emergency action, catching problems early gives you options. You can schedule repair during planned downtime, arrange a loaner or spare, and avoid the premium cost of emergency service.
Professional PLC Repair at Flexa Systems
We provide expert PLC repair services for all major brands including Allen-Bradley, Siemens, Mitsubishi, Omron, and more. Our repair process includes:
- Complete diagnostic evaluation
- Component-level repair (not just board swaps)
- Full functional testing with program verification
- 2-year warranty on all repairs
- Program backup and restoration
Don't wait for a complete failure. If your PLC is showing any of these warning signs, request a free evaluation or call (254) 254-0005. We'll help you plan a repair before it becomes an emergency.