Intermittent fault that's hard to reproduce
Something fails occasionally — random trips, dropouts, or stoppages — but works fine when you go to look at it. The classic 'can't fault it on the bench' problem.
Safety first
Intermittent faults can cause unexpected operation. Stay alert for sudden movement/energising. Apply isolation whenever you work hands-on.
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
Loose / high-resistance connection
Most likelyA loose terminal that makes and breaks with vibration or temperature is the most common intermittent.
- 2
Temperature-dependent fault
#2A component or joint that fails only when hot (or cold) appears and disappears with conditions.
- 3
Vibration / movement-related
#3A cable flexing on a moving part, or a connector working loose with vibration.
- 4
Environmental (moisture, contamination)
#4Damp or contamination causes leakage/faults only under certain conditions.
- 5
Electrical noise / event-triggered
Least likelyA fault triggered by another event (a large load switching, a specific machine cycle).
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 ModeLog occurrences with timestamps and conditions (load running, weather, machine step) to find a correlation.
A pattern linking the fault to a condition or event.
A correlation narrows the cause group — target it.
No pattern yet — apply provocation tests (heat/vibration/wiggle).
View all expected readings at once
Fault-finding flowchart
The same logic as a decision tree.
- 1start
Intermittent fault
→ step 2 - 2decision
Does logging conditions reveal a correlation?
Yes→ step 3No→ step 4 - 3result
Correlation found — target that cause group.
- 4decision
Does provocation (heat/vibration/wiggle) reproduce it?
Yes→ step 5No→ step 6 - 5result
Localised the marginal point — repair it.
- 6result
Use data-logging/monitoring to capture the event.
Common mistakes apprentices make
- Replacing parts at random hoping to get lucky.
- Not logging conditions to find a correlation.
- Giving up because it 'works fine now'.
- Overlooking connections in favour of components.
When to stop & escalate
Persistent intermittents that resist provocation and logging may need specialist monitoring or a planned approach with the wider team. Document everything so the next person isn't starting cold.
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
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.
VSD nuisance tripping with no obvious cause
The drive trips intermittently with codes that don't seem to match the conditions — random faults, hard to reproduce, often noise- or connection-related.
Loose connection overheating (discolouration / smell)
A terminal or connection is overheating — discoloured insulation, a burning smell, or heat you can feel — a common cause of nuisance faults and a real fire risk.