Industrial troubleshooting training: find the fault, then prove it

25 of the units are graded or interactive practice on simulated equipment rather than reading:

• Fix 01: Motor won’t hold: A start button runs the motor only while it is pressed. Repair the program so the motor remembers it was started.
• Fix 02: STOP is ignored: The motor holds itself on but nothing in the program reads STOP. Make STOP break the hold and let a fresh START run it again.
• Fault 1: Photoeye polarity reversal: A dark photoeye on a pre-wired panel. Measure the supply at the sensor, spot the crossed leads and submit your diagnosis.
• Fault 4: Broken +24 V supply wire to the PLC: The power supply is lit and the PLC is dark. Walk the rail from the source until the voltage disappears.
• Fix 03: Timer never times out: A two-second warning delay never finishes because the timer is fed by a momentary button. Keep the countdown alive and let STOP cancel it.
• Fix 04: Counter won’t reset: A packing station counts three boxes but the reset button does nothing. Clear the count so the next batch starts from zero.
• Fix 05: Two rungs, one output: A cooling fan obeys only one of its two requests because two rungs write the same output. Let one place decide the fan.
• Fix 06: Latch won’t release: An oven trip is remembered but can never be released. Make the reset wait for the cause to be gone and refuse it while the oven is hot.
• Fault 2: Relay contact card swap: A safety-OK indicator that never comes on, with the relay coil de-energised as it should be. Find the swapped contact.
• Fault 3: Rail short at the terminal block: The contactor never pulls in and the load side is dark. Probe around the terminal block to find the bridged rail.
• Fault 5: Broken 0 V return wire on the contactor coil: The PLC commands the contactor and nothing engages. Prove both sides of the coil to find the open return.
• Fault 6: Broken signal wire between photoeye and PLC: The sensor LED is on and the PLC input never reacts. Compare the two ends of the signal wire.
• Fault 7: NPN photoeye polarity reversal: An input stuck high with an empty conveyor. Decide whether the sensor leads are crossed or the sensor type is wrong.
• Fault 8: Relay contact card swap, alarm stuck on: An alarm beacon on since power-up with a healthy safety chain. Trace the relay contacts that were fitted the wrong way round.
• Fault 9: Blown 24 V control fuse: The supply indicators are healthy and the 24 V rail is dead. Prove voltage on both sides of the fuse.
• Fault 10: High-resistance solenoid terminal: A valve that chatters and measures low only while energised. Use a voltage-drop test under load.
• Fault 11: Thermal overload trip: The control supply is healthy and the contactor cannot energise. Trace the series chain to the open protective contact.
• Fault 12: Welded safety output contact: A light curtain is interrupted yet the contactor path stays continuous. Prove whether the relay output actually opens.
• Fault 13: Pt100 open compensation lead: An over-range temperature channel with a sensor-break diagnostic. Compare continuity on the three RTD conductors.
• Fault 14: Reversed thermocouple polarity: A temperature that falls as the process heats up. Verify polarity at the input card against the sensor type.
• Fault 15: Analog valve feedback drift: A valve display at 50 percent and a PLC reading consistently high. Separate wiring loss from a calibration offset.
• Fault 16: RS-485 terminator disconnected: Modbus that works slowly and fails at speed on long runs. Check end-to-end termination on the de-energised bus.
• Fault 17: VFD STO channel 1 open: The guard is closed and the drive stays inhibited. Compare both safety output channels at the relay and at the drive.
• Fault 18: Intermittent IO-Link C/Q conductor: A powered sensor that drops out when the machine vibrates. Find the conductor that only fails under movement.
• Fault 19: Load cell signal pair reversed: A weighing channel that reads backwards under load. Verify the millivolt signal pair against the transmitter.