Diagnosis Help
Intermittent Electrical Faults — How to Help Your Mechanic Find a Ghost Fault
An electrical fault that comes and goes is one of the hardest things to fix — because it is rarely there when someone is testing for it. The single most useful thing you can do is keep good notes. This guide shows you what to record.
This guide is about helping diagnosis, not doing it yourself. Observation and notes are safe; probing live circuits is not.
Why It’s Hard
Why intermittent faults are difficult
A fault that is always there can be tested and traced. A fault that appears only sometimes is harder, because it may have gone again by the time anyone connects test equipment. Several things make intermittent electrical faults behave this way:
- The fault disappears before anyone can test it
- Vibration while driving can temporarily restore a poor connection
- Heat expands components and changes a failing part’s behaviour
- Cold can make a failing component misbehave until it warms up
- Damp and rain can cause a poor connection to fail temporarily
- Wiring movement shifts a loose or corroded joint in and out of contact
- A failing relay may work sometimes and stick other times
- Voltage changes — low battery or charging — can trigger faults only under certain conditions
- Module or network faults can come and go without a clear pattern
- An intermittent sensor signal can be fine on a test drive but fail on the road
This is why intermittent faults can take more than one visit to confirm. See our auto-electrical diagnosis guide for the broader process.
Fault Log
Keep a fault log
The more precisely you can describe the pattern, the shorter the diagnostic time is likely to be.
Each time it happens, note down:
- Date and time of the fault
- Outside temperature
- Wet or dry weather
- Cold start or a warm engine
- Vehicle speed, or whether it was stationary
- Whether the engine was cranking, running, or switched off
- Which dashboard lights appeared
- Any sounds — clicking, buzzing, relays
- Which electrical systems stopped working
- Whether switching the vehicle off and back on changed anything
- Recent repair, battery or electrical work
- A photo or short video, if it is safe to take one
Even one or two clear entries can reveal a pattern — “only on cold wet mornings”, “only after 20 minutes of driving”, “only when the tank is low” — that points straight at the likely cause.
“No Fault Found”
“No fault found” doesn’t always mean nothing is wrong
A common and frustrating result with an intermittent fault is “no fault found” — the vehicle was tested, and at that moment everything behaved normally. That does not mean the fault is imaginary. It means the conditions that trigger it were not present during the test.
This is exactly where a fault log helps. If you can describe that it only happens in the cold, or only after a long run, or only in the wet, the next test can be aimed at reproducing those conditions instead of testing at random.
Why It Helps
Why good symptom information shortens diagnostic time
Diagnostic time costs money, and intermittent faults can eat a lot of it. Without a pattern, testing has to cover many possibilities. With a clear pattern, testing can focus on the circuit or condition most likely to be involved — which is faster and usually cheaper.
A scanner does not always find an intermittent fault. If the fault is not active during the code read, there may be nothing stored, or only a vague code that does not point to the real cause. Live data and the symptom pattern together are usually more useful than a code read alone. See vehicle electrical diagnostics.
FAQ
Short FAQ — Intermittent Faults
Because it is not there when someone tests it. Faults that come and go — only in the cold, only in the wet, only after a long drive — often disappear before testing can catch them. That is why detailed notes about exactly when and how it happens are so valuable.
No. It usually means the fault was not present at the moment of testing, not that it does not exist. A clear symptom pattern can help someone reproduce the conditions or test the right circuit next time.
Not always. A scanner reads stored codes and live data, but if the fault is not active during the test there may be nothing to find. Good symptom information is often more useful than a code read alone for intermittent faults.
Keep a fault log. Note exactly when it happens, the conditions, and what the vehicle does. The more precisely you can describe the pattern, the shorter the diagnostic time is likely to be.
Staying safe
- Do not probe live circuits or wiring to “catch” a fault yourself
- Do not wiggle or disturb wiring around airbag, fuel or high-current systems
- If a fault appears while driving, prioritise stopping safely over investigating it
- Observation and notes are safe and genuinely useful — leave the testing to someone equipped
If you’re in East Lancashire
Still not sure what the symptoms mean?
If you’re in Blackburn, Darwen, Accrington or the surrounding area, describe what the vehicle is doing and we can help you work out the next sensible step.