Proximity sensors (inductive, capacitive, photoelectric and magnetic) are everywhere on modern lines, and a surprising number of "PLC faults" and "machine won't run" faults turn out to be a single failed or misaligned sensor. This guide gives a safe, structured approach.

Safety

Follow site isolation procedures. Safety devices must never be bypassed or overridden — if the sensor in question is a safety-rated device (e.g. a light curtain or safety interlock), follow the machine's safety system documentation, not this general guide.

Symptoms

  • Machine stops or won't start — permissive input not seen.
  • Intermittent stoppages — sensor flickers under vibration.
  • Persistent "present" or "absent" regardless of the target.
  • Fault that appears only after washdown or in certain weather/temperature.

A diagnostic sequence

  1. Physical alignment — is the sensor at the correct distance and angle to its target? Has it shifted?
  2. Target detection — is the target the right material? (Inductive sensors only see metal; capacitive sense more; photoelectrics depend on reflectivity.)
  3. Contamination — dust, grease, water or product on the sensing face is a very common cause, especially in food, feed and washdown environments.
  4. Wiring — continuity, correct common, no chafing, connector tight.
  5. Power — correct supply voltage at the sensor.
  6. Output / PLC input — does the sensor's indicator change with the target? Does the PLC input change with it?
  7. Intermittent operation — flex the cable, watch for change; check ferrules, crimps and vibration loosening.
  8. Replacement — match the sensor type, voltage, output type (PNP/NPN), sensing range and connection; do not substitute indiscriminately.

A useful check

If a sensor is suspected, swap the sensor with a known-good one of the same type where safe. If the fault follows the sensor, the sensor is the problem; if it stays with the position, the issue is alignment, target or wiring there.

Key points

  • Clean the sensing face before condemning the sensor — contamination is the leading cause.
  • Confirm the output type matches the PLC input (PNP vs NPN) before replacing.
  • Capture the make/model and the fault so the repeat occurrence is instantly recognisable.