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.
Safety first
Intermittent inputs can cause unexpected machine behaviour. Be ready for unplanned movement while fault-finding. 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.
Full detail — causes, the why, and common mistakes.
Likely causes
Ranked from most to least likely.
- 1
Marginal sensing gap / vibration
Most likelyIf the gap is right at the edge of range, vibration or slight movement makes detection come and go.
- 2
Damaged or chafing cable
#2A cable flexing on a moving part, or chafed insulation, makes intermittent contact.
- 3
Loose terminal / connector
#3A loose terminal or a dirty connector pin causes drop-outs as things move or warm up.
- 4
Electrical noise / interference
#4Noise coupled into the signal (poor screening, running beside power cables) causes false triggers.
- 5
Sensor near end of life
Least likelyAn aging sensor can become marginal and trigger unreliably.
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.
Check the sensing gap and look for vibration/movement at the detection point.
A comfortable gap within range, stable at the trigger point.
Gap fine — wiggle-test the cable and connectors.
Marginal gap or vibration — adjust the gap / secure the mounting.
View all expected readings at once
Fault-finding flowchart
The same logic as a decision tree.
- 1start
Sensor intermittent
→ step 2 - 2decision
Is the sensing gap comfortable and the mounting stable?
Yes→ step 3No→ step 4 - 3decision
Does wiggle-testing the cable/terminals change the input?
Yes→ step 5No→ step 6 - 4result
Marginal gap / vibration — adjust gap, secure mounting.
- 5result
Damaged cable or loose connection — repair it.
- 6result
Consider noise/routing/screening or an aging sensor — improve or replace.
Common mistakes apprentices make
- Chasing the controller when an input is dropping out at the sensor.
- Running signal cables alongside power and getting noise.
- Setting the gap right at the edge of the sensing range.
- Not wiggle-testing the cable on moving machinery.
When to stop & escalate
Intermittent faults that can't be reproduced should be logged with timestamps to correlate with events. Cable routing/screening fixes on a larger installation may need planning.
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.
Related faults
Limit switch or proximity sensor not being detected
A limit switch or proximity/photo sensor isn't registering — the machine doesn't stop at position, the input never makes, or the sensor LED looks wrong for the target's position.
Proximity sensor permanently on (always made)
A proximity sensor reads 'detected' all the time — its output stays on even with no target present, so the controller always sees the input made.
Encoder feedback fault (position/speed wrong)
Position or speed feedback from an encoder is wrong — counts drift, position is lost, or a drive faults on encoder loss, so motion control misbehaves.
Learn the theory
How the gear and circuits behind this fault actually work.