Short answer

First separate the sensor from the PLC input and observe the output with a suitable test load or meter. If the output remains active without a target, check for an output short or a wiring mistake before changing the sensor.

PNP sensor to PLC input wiring diagramA three-wire PNP sensor sends positive voltage to a PLC input.PNP / SOURCING OUTPUTSENSOR3-wire DCPLC INPUTsinking / common −I0.0+24 V / brownsignal / black0 V / blue Original diagram · Signal Field Guide / field note troubleshooting

What to look for

Confirm the supply polarity and the 0 V reference. A floating or shared common can make the input state look permanently true even when the sensor output is behaving as designed.

If the electrical checks pass, inspect target distance, hysteresis, and mounting. A metal target close to the sensing face can keep a proximity sensor inside its active zone.

FIELD CASE / REASONING PATH

When the symptom appears in the plant

A bottling line kept jamming because a bottle present sensor remained triggered even when the conveyor was empty. The technician cleaned the lens, but the LED stayed on. Upon measuring the output, they found 24V regardless of target presence. The sensor's internal solid-state output had failed in a shorted condition due to a previous wiring mistake that exposed it to a voltage spike without a flyback diode on the inductive relay coil it was driving.

DIAGNOSTIC SEQUENCE

Five checks before replacement

  1. Disconnect the sensor output wire from the load and measure the voltage; if it still reads 24V without a target, the sensor is internally shorted.
  2. Check the physical environment: look for extreme dust, condensation, or a highly reflective background surface causing a false positive on optical sensors.
  3. Verify the load (relay or PLC input) is not damaged or shorted to 24V, backfeeding voltage into the sensor output.
  4. Inspect the sensor alignment and sensitivity adjustment (potentiometer); it may be turned up too high, detecting background objects.
  5. Check for electrical noise or voltage spikes on the 24V supply that could falsely trigger or permanently damage the solid-state output.
PRACTICAL NOTES

Solid-state outputs (like those in PNP sensors) often fail closed (shorted).

Always use flyback diodes or surge suppressors when a sensor drives an inductive load like a mechanical relay or solenoid.

For optical sensors, ensure the 'Light On / Dark On' switch is set correctly for the application logic.

BEFORE YOU REPLACE THE PART

Bench checklist

  • Confirm the supply at the device terminals.
  • Confirm the meter is in the correct mode and position.
  • Confirm the input range matches the signal type.
  • Confirm the reference/common is intentional.
  • Record the reading before changing the wiring.

Reference families

  • Sensor Troubleshooting Manuals
  • Control Engineering Best Practices

Always verify the current edition and the exact device manual for the installation.

READ THE PRIMARY MATERIAL

Official references

These links point to standards bodies, industry organizations, or manufacturer documentation. Standards landing pages may require purchase or institutional access.

This note is an educational reference, not a substitute for the equipment manual, site procedures, or qualified electrical work.