Photoelectric sensor not detecting
A photo-eye (through-beam, retro-reflective, or diffuse) isn't detecting reliably — it misses the target, or the output doesn't change when something blocks/enters the beam.
Safety first
Confirm what the sensor controls before testing by blocking the beam — it may start or stop machinery. Isolate before handling wiring.
Isolate, lock out / tag out, and prove dead before working unless a live test is specifically required, authorised, and carried out under proper supervision. Always follow local regulations, your site procedures, and the equipment manufacturer's documentation.
Apprentice mode
Full walk-through — the why, what readings mean, mistakes & safety.
Likely causes
Ranked from most to least likely.
- 1
Dirty lens / reflector or blocked beam
Most likelyDust, oil, or condensation on the lens or reflector weakens the beam so it won't switch reliably.
- 2
Misalignment
#2Emitter/receiver or sensor/reflector knocked out of alignment so the beam doesn't return/reach.
- 3
Wrong sensing mode for the target
#3A shiny, dark, or transparent target can defeat a diffuse sensor; the mode/sensitivity is wrong for it.
- 4
Sensitivity/gain set wrong
#4Gain set too low to detect, or too high so it sees through the target.
- 5
Sensor or wiring fault
Least likelyA failed emitter/receiver or a wiring fault stops detection.
Reports are saved on this device to reflect what you actually find.
Testing sequence
Work through one test at a time. Expected reading and what each result means.
On the tools? Job Mode shows one test at a time with thumb-sized buttons — your place and readings carry straight over.
Enter Job ModeClean the lens/reflector and check the alignment/stability indicator if fitted.
Strong, stable beam/margin indication after cleaning/aligning.
Beam strong — check the target/mode and sensitivity.
Weak/unstable — clean and realign until the margin is good.
View all expected readings at once
Fault-finding flowchart
The same logic as a decision tree.
- 1start
Photo-eye not detecting
→ step 2 - 2decision
After cleaning/aligning, is the beam/margin strong?
Yes→ step 3No→ step 4 - 3decision
Does the output switch cleanly with the target and correct mode?
Yes→ step 5No→ step 6 - 4result
Clean and realign until the margin is good.
- 5result
Detecting — confirm consistency across the range.
- 6decision
Are supply, output and wiring all sound?
Yes→ step 7No→ step 8 - 7result
Adjust gain / change sensing mode to suit the target.
- 8result
Failed sensor or wiring — repair/replace.
Common mistakes apprentices make
- Not cleaning the lens/reflector first — most photo-eye faults are contamination.
- Using a diffuse sensor on a shiny or transparent target.
- Setting gain so high it sees through the object.
- Knocking alignment and not re-checking the margin indicator.
When to stop & escalate
Persistent alignment loss may indicate a mounting/vibration issue to address. A failed sensor should be replaced like-for-like with the correct sensing mode.
If you're past your competence, authorisation, or the safe limits of the job — stop and hand it on. There's no fault worth getting hurt over.
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Related faults
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Sensor intermittent / drops out randomly
A sensor works most of the time but drops out or false-triggers intermittently, causing random stops, miscounts, or sequence faults that are hard to pin down.
Analogue sensor (4-20mA / 0-10V) reading wrong
An analogue sensor (pressure, level, temperature) gives a wrong or fixed reading — stuck at zero, pinned at full scale, or simply not matching reality.
Learn the theory
How the gear and circuits behind this fault actually work.