Intermittent Electrical Faults — Why Your Car “Sometimes” Won’t Start (And Why That’s the Hardest Problem to Fix)
“It started fine yesterday. Today it won’t turn over.”
If you’ve ever said that sentence, you’ve experienced the most frustrating type of car problem: the intermittent electrical fault. It’s the fault that never appears when the mechanic is looking. It’s the warning light that vanishes the moment you drive into the garage. And it’s the repair that can cost far more than it should — not because the part is expensive, but because finding the fault is the hard part.
At Homerton Car Repairs, we diagnose electrical faults every week. And we’ll be honest with you: intermittent faults are the most time-consuming, most misunderstood, and most misdiagnosed problems in the entire automotive industry. This guide explains why — and what you can do about it.
⚡ Why Intermittent Faults Are Different
A permanent fault is predictable. Your brake light doesn’t work, and it stays broken. A mechanic traces the circuit, finds the break, fixes it, done.
An intermittent fault is unpredictable. It depends on temperature, vibration, moisture, load, or a combination of all four. It might only happen when the car is cold. Or only when it’s hot. Or only when you go over a specific bump on a specific road. Or only on Tuesdays when it’s raining (we wish we were joking).
The result: The fault is not “live” when the car is in the workshop. The mechanic can’t reproduce it. The scan tool shows nothing. And you’re left with a car that “sometimes” works and a garage that can’t find anything wrong.
Expert insight: The trade habit is often to “guess, fit, and hope” — replacing sensors and modules until the problem goes away. This is expensive and unnecessary. At Homerton Car Repairs, we use a systematic diagnostic approach, not parts-swapping .
🔍 The 6 Most Common Causes of Intermittent Electrical Faults
1. Body Control Module (BCM) Failure
The BCM is the computer that controls your windows, lights, locks, wipers, and alarm. It’s also one of the UK’s top five causes of breakdowns, with AA data showing BCM-related breakdowns have doubled to more than 36,000 per year since 2020 .
Symptoms:
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Windows that work “sometimes”
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Lights that flicker or stay on after locking
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Central locking that randomly fails
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Battery draining overnight for no apparent reason
Why it’s hard to diagnose: The BCM is a computer. It doesn’t “break” like a mechanical part — it develops software glitches, corrupted memory, or water-damaged circuit boards that behave unpredictably.
2. Alternator Faults (Not Always Obvious)
Most drivers know that a failed alternator means a flat battery. But an alternator can partially fail — charging correctly at idle but dropping voltage under load, or vice versa.
Symptoms:
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Dimming headlights at idle
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Battery warning light flickering (not solid)
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Multiple unrelated warning lights appearing randomly
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Radio or infotainment system resetting itself
Diagnostic difficulty: A standard battery test won’t catch it. You need live data monitoring while the car is running under load — something most fast-fit centres don’t do.
3. Wiring Loom Chafing and Corrosion
This is the most common physical cause of intermittent faults, especially on older cars or vehicles that spend a lot of time in stop-start city traffic.
What happens: Wiring rubs against a metal bracket, wears through its insulation, and creates an intermittent short circuit. It only misbehaves when the car vibrates at a certain frequency or temperature.
Where to look:
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Behind the dashboard (vibration from rough roads)
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In the engine bay (heat cycling and oil contamination)
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At door hinges (constant flexing)
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Under the bonnet near the battery (acid corrosion)
British weather warning: Cold, damp conditions accelerate connector corrosion. We see a spike in electrical faults every February and March .
4. Faulty Relays and Fuses
A relay is an electrical switch. When it fails, it can fail intermittently — working 90% of the time and failing the other 10%.
Symptoms:
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A circuit that works when you tap the fuse box
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Accessories that work in warm weather but not cold
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A single function (e.g., fuel pump) that cuts out randomly
Why it’s frustrating: Relays are cheap (£5–£15), but finding which one is failing requires circuit-by-circuit testing — time-consuming but far cheaper than guessing.
5. Crankshaft and Camshaft Position Sensors
These sensors tell the engine computer when to fire the spark plugs and inject fuel. When they begin to fail, they often do so only when hot.
Symptoms:
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Car starts fine when cold, won’t restart when warm
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Engine cuts out at low speed (e.g., approaching a junction)
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No warning light (these sensors often fail without logging a code)
Diagnostic difficulty: The sensor may test perfectly when cold but fail at operating temperature. Only live data logging during a road test will catch it.
6. Water Ingress and Earth Point Corrosion
Water gets into places it shouldn’t — through a blocked sunroof drain, a perished door seal, or a cracked light cluster. It then corrodes earth points (the metal connections that complete electrical circuits).
Symptoms:
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Multiple unrelated electrical faults
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Lights that glow dimly when they should be off
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Strange behaviour after rain or a car wash
Where to check:
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Under the carpet (water pooling from a blocked drain)
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Behind the boot trim
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Under the bonnet at factory earth points
🛠️ Why Intermittent Faults Cost More to Fix
| Factor | Permanent Fault | Intermittent Fault |
|---|---|---|
| Diagnosis time | 30–60 minutes | 2–6 hours (or more) |
| Parts cost | Usually the main cost | Often low — labour is the cost |
| Reproducibility | Always present | May not appear in workshop |
| Diagnostic approach | Visual inspection + scan | Live data, road testing, circuit tracing |
| Risk of misdiagnosis | Low | High (without systematic testing) |
The honest truth: An intermittent fault that takes four hours to find might cost £240 in labour — even if the replacement part is only £15. That’s not a rip-off. That’s the cost of skilled diagnostic work.
Red flag alert: If a garage says “we’ll just replace this sensor and see if it fixes it,” walk away. That’s guessing, not diagnosing. A proper garage will test before replacing .
🧪 How Homerton Car Repairs Diagnoses Intermittent Faults
We don’t guess. We follow a systematic process:
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Customer interview first — When does it happen? Hot or cold? Wet or dry? What were you doing? This is the most valuable diagnostic tool we have.
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Live data monitoring — We connect diagnostic equipment and monitor sensor readings in real time, often during a road test.
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Circuit tracing — We test individual circuits with a multimeter and oscilloscope to find voltage drops and resistance changes.
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Wiggle testing — We physically move wiring looms and connectors while monitoring readings to expose chafing and loose connections.
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Thermal testing — We use heat guns and freeze spray to simulate temperature-related failures.
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Photo and video evidence — We document everything we find, so you can see exactly what was wrong.
Our diagnostic fee: £60 (waived if you proceed with the repair at our garage). Most intermittent faults are found within 2–3 hours.
🚗 Why Homerton and Hackney Drivers Are Prone to Electrical Faults
City driving creates perfect conditions for electrical problems:
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Constant vibration from potholed roads accelerates wiring chafing
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Stop-start traffic causes extreme heat cycling in the engine bay
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Short journeys prevent the battery from fully recharging
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Damp, polluted air corrodes connectors faster than rural driving
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Older vehicles (common in London) have more brittle wiring and ageing modules
We know these conditions intimately. Our diagnostic approach is built for real East London driving, not laboratory conditions.
⚠️ When to Book a Diagnostic Check
Book an appointment if you experience:
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✅ Any warning light that appears and disappears
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✅ A car that won’t start “sometimes” (especially when hot)
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✅ Multiple unrelated electrical faults at once
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✅ A battery that drains overnight with no obvious cause
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✅ Accessories that work intermittently
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✅ Any electrical smell (burning plastic) — stop driving immediately
🔧 Don’t Let a “Sometimes” Problem Become a “Never” Problem
Intermittent faults don’t fix themselves. They get worse. A wiring fault that causes one flickering light today can cause a complete electrical failure — including loss of power steering or brake lights — next month.
Book your electrical diagnostic check today. We’ll find the fault, explain it in plain English, and give you a fixed-price quote before we touch a thing.
📍 Homerton Car Repairs — Serving Homerton, Hackney & East London
