PLC Fault Finding & Troubleshooting Procedures
The following tables list some PLC Fault Finding common problems and troubleshooting procedures for the PLC systemin the event of faulty operation.
Symptom Troubleshooting and Corrective Actions for PLC system
PLC Fault Finding & Troubleshooting Procedures Corrective Actions for PLC system
All LEDs are OFF
1. Check the power supply wiring.
2. Check If the power supplied to the PLC control units is in the range of the rating.
3. Be sure to check the fluctuation in the power supply.
4. Disconnect the power supply wiring to the other devices if the powersupplied to the PLC control unit is shared with them.If the LEDs on the PLC control unit turn ON at this moment, the
capacity of the power supply is not enough to control other devices
as well. Prepare another power supply for other devices or increase
the capacity of the power supply.
5. If the POWER LED still does not light up when the power is on after
the above corrective actions, the PLC should be sent back to the
dealer or the distributor whom you purchased the product from
ERROR LED is
1. If the ERROR LED is flashing, the problem may be an invalid
commands, communication error, invalid operation, or missing
instructions, error indication is given by self-checking function and
corresponding error code and error step are stored in special
registers. The corresponding error codes can be read from the
WPLSoft or HPP. Error codes and error steps are stored in the
following special registers.
Error code: D1004
Error step: D1137
2. If the connections between the PLC are failed and the LED will flash
rapidly, this indicates the DC24V power supply is down and please
check for possible DC24V overload.
3. The LED will be steady if the program loop execution time is over thepreset time (set in D1000), check the programs or the WDT (Watch Dog Timer). If the LED remains steady, download user program again and then power up to see if the LED will be OFF. If not, please check if there is any noise interference or any foreign object in the PLC.
When input indicator LEDs are OFF,
1. Check the wiring of the input devices.
2. Check that the power is properly supplied to the input terminals.
3. If the power is properly supplied to the input terminal, there is
probably an abnormality in the PLC’s input circuit. Please contact
4. If the power is not properly supplied to the input terminal, there is
probably an abnormality in the input device or input power supply.
Check the input device and input power supply.
When input indicator LEDs are ON,
5. Monitor the input condition using a programming tool. If the input
monitored is OFF, there is probably an abnormality in the
PLC’s input circuit. Please contact your dealer.
6. If the input monitored is ON, check the program again. Also, check
the leakage current at the input devices (e.g., two-wire sensor) and
check for the duplicated use of output or the program flow when a
control instruction such as MC or CJ is used.
7. Check the settings of the I/O allocation.
When output indicator LEDs are ON,
1. Check the wiring of the loads.
2. Check if the power is properly supplied to the loads.
3. If the power is properly supplied to the load, there is probably an
abnormality in the load. Check the load again.
4. If the power is not supplied to the load, there is probably an
abnormality in the PLC’s output circuit. Pleas contact your dealer.
When output indicator LEDs are OFF,
5. Monitor the output condition using a programming tool. If the output monitored is turned ON, there is probably a duplicated output error.
6. Forcing ON the output using a programming tool. If the output
indicator LED is turned ON, go to input condition check. If the output
LED remains OFF, there is probably an abnormality in the PLC’s
output circuit. Please contact your dealer.