"Gate not closed" and random stops — position sensors and the 24 V supply
Symptom
The plant stops with "gate not closed" or "door open" while it is closed, stops at random, or a position signal flickers.
Likely causes
- Sensor moved or loose, or sensing gap too large.
- Sensor covered in concrete or dust.
- Reed sensor slid along the cylinder.
- Cut or crushed cable, or water in the connector.
- Failing 24 V DC supply, loose wire, or earth fault on the 24 V circuit.
Most likely and cheapest to check first.
Checks
- Look at the real gate or door position first.
- Check the sensor LED changes state at each end of travel, and that the PLC input lamp follows.
- Check the sensor fixing screws and gap; mark its position before removing it.
- Have the 24 V supply voltage measured under load.
Fix
- Clean the sensor; reset the gap well inside the rated sensing distance.
- Replace with exactly the same type - PNP or NPN, NO or NC, voltage, sensing distance, connector.
- Repair the cable; dry and seal the connector.
- Replace a failing 24 V supply; tighten terminals.
Stop the equipment if
Someone proposes to bridge a sensor input to keep producing. A bridged input lets the plant run blind.
Parts you may need
Never bridge a sensor
A gate shown closed while it is open overfills a scale. A mixer door shown closed while open dumps concrete onto a truck that is not there. Fix the sensor, never the signal.
FIELD TIPKeep two spare proximity sensors of each type used on gates and doors, and a spare 24 V power supply. Together they cost less than one lost shift.
⚠ Every check inside a machine starts with full lockout. Safety disclaimer