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.
Quick reference: PNP 3-Wire Sensor Output States, Failure Modes & Multimeter Diagnostics
| Sensor Operating State | Output Wire (Black) Condition | DMM Reading (to 0V Blue) | Target Condition | Diagnostic Root Cause |
|---|---|---|---|---|
| Normal OFF State | Output transistor non-conducting | < 0.8 V DC (under load) | No metallic target within range | Normal operation; minimal off-state semiconductor leakage |
| Normal ON State | Output transistor conducting | 22.5 V – 23.6 V DC | Valid target within sensing range | Normal operation; 0.4 V to 1.5 V internal transistor saturation drop |
| Internally Shorted ON | Permanent junction short-circuit | 23.8 V – 24.0 V DC constant | Target removed or distant | PNP output stage destroyed by inductive surge spike or external load short |
| Mechanical / Collar Over-sensing | Output transistor conducting | 23.0 V – 23.5 V DC | Non-flush sensor recessed in steel | Unshielded sensor face detecting metal mounting bracket as continuous target |
| High-Z DMM Ghost Voltage | High-impedance float (>10 MΩ) | 12.0 V – 18.0 V DC (unloaded) | No target present | Multimeter reading off-state leakage; drops to 0.0 V when loaded with 10 kΩ resistor |
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.
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.
The 10 MΩ Multimeter Off-State Leakage False Alarm
A standard digital multimeter has an input impedance of 10 MΩ. Solid-state sensors (PNP transistors, inductive proximity, optical switches) exhibit a tiny quiescent off-state leakage current (typically 0.1 mA to 0.8 mA). When measuring an open, disconnected output terminal with a 10 MΩ meter, that sub-milliamp leakage current develops an open-circuit voltage reading between 12 V DC and 22 V DC. Technicians routinely mistake this ghost voltage for a shorted, stuck-on sensor. Before condemning the sensor, place a 10 kΩ 1/2W resistor across the black output wire and the blue 0V wire, or measure while connected to the PLC sinking input. If the voltage collapses below 1.5 V DC under load, the sensor output stage is completely healthy.
Five checks before replacement
- Disconnect the sensor output wire (black) from the PLC input terminal and measure voltage across black and blue (0V) with the target absent.
- If the unloaded wire still reads near supply voltage, place a 10 kΩ burden resistor across black and blue; if it remains at 24V, the PNP output transistor is internally shorted.
- Inspect the physical mounting face: verify an unshielded (non-flush) sensor is not recessed into a metal plate, causing the housing to act as a permanent target.
- Check the sensor's optical sensitivity potentiometer or teach-in setting; excessive gain can detect conveyor backgrounds, dust accumulations, or ambient light reflections.
- Measure the 24V DC power rail with a multimeter set to min/max peak capture to verify inductive loads (coils/brakes) are not generating reverse voltage transients without flyback protection.
Solid-state PNP outputs fail closed (shorted) far more frequently than open when subjected to overcurrent or inductive flyback spikes.
Always install a 1N4007 flyback diode across DC relay coils driven directly by sensor outputs; reverse inductive kickback can exceed 200V in 5 microseconds.
For optical sensors, verify the selector switch is not inadvertently flipped from Light-ON to Dark-ON operation.
Bench checklist
- Disconnect the Black output wire from the PLC input and measure voltage to Blue 0V with a high-impedance meter.
- If output stays high with no load, check for an internal transistor short or water ingress inside the sensor housing.
- Verify that the sensing face is clean: remove metallic chips, swarf, or conductive slurry buildup.
- Check target mechanical clearance: ensure target fully retracts beyond the release distance (hysteresis threshold).
- Confirm whether the sensor is configured as Normally Open (NO) or Normally Closed (NC) in the part number.
Reference families
- IEC 60947-5-2 Standard
- Sensor Output Circuit Protection Guidelines
Always verify the current edition and the exact device manual for the installation.
Official references
These links point to standards bodies, industry organizations, or manufacturer documentation. Standards landing pages may require purchase or institutional access.
- PNP vs NPN: What Makes These Outputs Different? Banner Engineering
Explains sourcing outputs, load placement, and PLC input matching.
- TM18 Series Sensors Datasheet Banner Engineering
Official datasheet with PNP wiring, ON/OFF behavior, overload, and leakage data.
- E3AS-HL150 / HL500 Instruction Sheet Omron
Official manual covering NO/NC inversion, hysteresis, short-circuit checks, and detection conditions.
- IEC 61131-2:2017 IEC
Official PLC digital-I/O standard landing page; the standard text may require purchase.