Most proximity sensor “failures” are not failed sensors. In the field the usual causes are, in order: the gap has drifted beyond the assured sensing distance, the target material is not what the sensor was sized for, the wiring polarity does not suit the PLC input, or electrical noise from a nearby drive is corrupting the signal. Work through them in that order before replacing anything.
Diagnose in this order
- 1. Power — measure at the sensor, not at the terminal rail.
- 2. Output — does the sensor switch at all? Watch the LED, then meter the output.
- 3. Gap and target — is the working gap inside 0.81 × Sn × reduction factor?
- 4. Interface — polarity, leakage current, input card common.
- 5. Environment — noise, temperature, vibration, contamination.
The twelve faults, and how to identify each
1. Sensor never operates — no LED, no output
Measure supply voltage at the sensor connector, with the machine running. A long cable run, a corroded terminal or an overloaded 24 V supply can drop several volts under load. Below the sensor’s minimum operating voltage the oscillator will not start. If the supply is good, check polarity: brown to +, blue to 0 V. Most sensors survive reversal, but they do not operate.
2. LED lights but the PLC never sees it
The sensor is working; the interface is not. Check whether you have a PNP sensor on a sourcing input card, or an NPN sensor on a sinking one — the two must be opposites. This is the single most common commissioning fault, and our guide to PNP versus NPN wiring explains how to confirm which you have.
3. PLC input reads permanently ON
With a two-wire sensor, this is almost always residual leakage current holding a high-impedance input above its OFF threshold. Fit a bleeder resistor across the input or move to a three-wire sensor. With a three-wire sensor, suspect a short between the output core and the supply, or an unshielded non-flush sensor detecting its own mounting bracket.
4. Intermittent — works cold, fails hot
This is the classic symptom of designing to the catalogue sensing distance instead of the assured distance. Rated distance Sn is a nominal figure; guaranteed detection only exists inside 0.81 × Sn, and that must be further multiplied by the reduction factor for the target material. Measure the actual gap and recalculate. Nine times in ten the gap is too large.
5. Detection distance is far shorter than the catalogue says
Check the target material first. Aluminium at a reduction factor of about 0.4 gives you well under half the rated range; copper less still. Then check target size — a target smaller than the reference square loses range roughly in proportion to area. Then check thickness and any plating.
6. Unshielded sensor latches on permanently
It is detecting its own mounting. A non-flush sensor needs a metal-free zone: a clear diameter of at least three times the barrel diameter, with the head standing proud of surrounding metal by at least twice Sn. Either move it out or fit a shielded sensor — see shielded versus unshielded mounting.
7. Two adjacent sensors chatter randomly
Mutual interference. Two inductive sensors mounted too close beat against each other and both outputs become unstable, typically worsening as they warm. Minimum spacing is twice the barrel diameter for shielded units and three times for unshielded. If the layout cannot be changed, specify units with different oscillator frequencies.
8. Random false triggers when a drive or contactor operates
Electrical noise. Confirm by operating the drive with the machine mechanically stationary — if the sensor still trips, it is not seeing a target. Route sensor cables away from VFD motor cables, never in the same trunking or parallel for long runs. Use screened cable with the screen earthed at the panel end only. Add a suppressor across any inductive load switched near the sensor.
9. Output is on but the load will not operate
Check load current against the sensor’s rated output current — typically 100–200 mA for a DC three-wire unit. Contactor coils and lamps often exceed it. Also check voltage drop, particularly with two-wire sensors in series, where the drops add and can starve the load. Our note on two, three and four-wire wiring covers the arithmetic.
10. Works, then dies after a few weeks
Look for mechanical and environmental causes. Vibration loosens locknuts and fatigues cable at the strain relief — flex the cable at the entry and see if the LED flickers. Coolant or washdown ingress past a degraded seal is next. Check the sensor’s IP rating against the actual environment, remembering that the connector is part of the seal.
11. Missing targets at speed
Every sensor has a maximum switching frequency, and the target must remain in the field long enough to be recognised. Calculate the dwell time: target length divided by line speed. If it approaches the sensor’s response time, you need a faster sensor, a longer target flag, or a different sensing position. See our note on proximity switches in high-speed counting.
12. Capacitive sensor stays on after the product has gone
Build-up on the sensing face or product clinging to the inside of a tank wall. The sensor is behaving correctly — high-dielectric material is still in its field. Reduce sensitivity so bulk product is needed rather than a film, and relocate to a point with flow across it. Details in our guide to capacitive sensors for level detection.
A bench test that settles most arguments
When a sensor is suspected, remove it from the machine and test it on a bench with nothing else connected:
- Apply rated supply — brown to +24 V, blue to 0 V.
- Leave the output (black) unconnected.
- Meter between black and blue for a PNP unit, or between brown and black for an NPN unit.
- Present a mild steel target, 1 mm thick, at least as wide as the sensing face.
- Move it in slowly and note the distance at which the output switches, then move it out and note where it releases. The difference is the hysteresis, normally 5–20% of the sensing distance.
If the switching distance is close to the catalogue figure on mild steel, the sensor is good and the problem is in the installation, the target, or the interface. That single test eliminates most of the guesswork — and it is worth doing before ordering a replacement.
Quick symptom index
| Symptom | Check first | Then |
|---|---|---|
| No LED, no output | Supply voltage at the sensor | Polarity, cable continuity |
| LED on, PLC sees nothing | PNP/NPN versus input card common | Output core continuity |
| PLC input always on | Two-wire leakage current | Sensor detecting its mounting |
| Intermittent, worse when hot | Working gap versus 0.81 × Sn | Supply voltage under load |
| Range much shorter than rated | Target material and size | Thickness, plating, coating |
| Random trips with drive running | Cable routing and screening | Earthing, load suppression |
| Misses fast targets | Dwell time versus response time | Target flag length |
| Fails after weeks in service | Cable at the strain relief | Seal, IP rating, vibration |
Frequently asked questions
How do I test a proximity sensor with a multimeter?
Power it at rated voltage with the output disconnected. For a PNP unit, measure between the black output and blue 0 V — it should read close to supply voltage when a target is present. For an NPN unit, measure between brown and black — the voltage should collapse when it operates. If the sensor switches correctly on a mild steel target at roughly the catalogue distance, the fault is elsewhere.
Why does my sensor work at commissioning but fail in summer?
Because the gap was set to the rated distance rather than the assured distance. Sensing range falls at elevated temperature and low supply voltage — that variation is exactly what the assured operating distance of 0.81 × Sn exists to cover. Reduce the gap or fit a larger sensor.
Can a proximity sensor be damaged by wiring it wrongly?
Most quality sensors have reverse-polarity and short-circuit protection and survive common mistakes, simply not operating until corrected. What does destroy them is connecting the output directly across the supply without a load, applying AC to a DC unit, or exceeding the rated supply voltage. Check the label before applying power to an unfamiliar sensor.
What causes a proximity sensor to trigger with nothing in front of it?
Three main causes: an unshielded sensor detecting its own mounting bracket, electrical noise coupled from a nearby drive or contactor, or — on capacitive units — build-up on the sensing face. Test by removing the sensor from the bracket and observing it in free air; if it releases, the mounting is the problem.
How long should an industrial proximity sensor last?
There is no wearing contact, so electronic life is long — many run for a decade or more. Practical life is set by the environment: cable flexing at the entry, seal degradation under washdown, abrasion in dusty plant, and thermal cycling. Sensors that fail early almost always fail mechanically or through ingress, not electronically.
Still chasing an intermittent sensor? Send us the part number, the target material and gap, and what the LED does when the fault occurs. Accent Controls has been supporting sensor installations across Indian industry since 1985, and our engineers can usually identify the cause from those three facts.