START-STOP NOT WORKING: CAUSES, SYMPTOMS AND SOLUTIONS (2025)
Does your start-stop system stop working just when you need it, such as in traffic jams or at traffic lights? From 2020 to 2025, the number of cases has increased on petrol, diesel, and hybrid vehicles. In this updated guide, we analyze the real causes , the most common symptoms , how to make a correct diagnosis , and the most effective solutions (with indicative costs).
Battery problem? Here's the complete guide.
Start-Stop is closely linked to battery health. Here's a guide to the best 2025 batteries with technical advice, types (AGM, EFB), and lifespan.
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⚙️ HOW THE START-STOP SYSTEM REALLY WORKS
Start-Stop is not a simple “turn off and on”: it is a system that works together with:
- EFB/AGM battery (much more robust than normal ones);
- Intelligent alternator (with energy recovery);
- IBS/BMS sensor (battery condition monitoring);
- Engine, climate, ABS and ESP control units;
- Hood, door, brake, clutch, temperature sensors.
To activate, the system must verify that ALL parameters are within normal limits. A single value outside the normal range deactivates it.
🔍 MOST COMMON CAUSES WHEN THE START-STOP DOESN'T WORK
From our experience with thousands of interventions on modern cars, these are the most frequent causes:
1) Weak or unsuitable service battery (80% of cases)
- battery no longer efficient;
- normal battery fitted in place of EFB/AGM;
- new battery but not “registered” in the control unit.
2) Faulty or uncalibrated IBS/BMS sensor
This sensor measures the battery's health. If it reads inconsistent values, it turns off the start-stop system to protect the engine.
3) Alternator/voltage regulator
- insufficient charge;
- charge too high;
- energy recovery not working.
4) Engine or battery temperature out of range
If it is too cold or too hot, the system deactivates automatically.
5) Air conditioning and heating “at full load”
If the air conditioning requires a lot of energy, the control unit prefers to keep the engine running.
6) Errors in the control units or external sensors
- direction of travel / ABS;
- hood sensor;
- clutch or brake microswitches;
- engine/air temperature sensors.
7) DPF status (diesel)
During regeneration , Start-Stop is automatically deactivated .
🚨 TYPICAL SYMPTOMS OF START-STOP NOT WORKING
- crossed out Start-Stop icon on the dashboard;
- message “Start-Stop function not available”;
- very strong engine fan (DPF regeneration in progress);
- climate that maintains high engine revs;
- start-stop that only works sometimes;
- engine that does not restart automatically but only with the button;
- battery that appears “charged” but is actually weak under stress.
🧪 HOW TO MAKE A CORRECT DIAGNOSIS (WORKSHOP METHOD)
Recommended checklist:
- Testing EFB/AGM batteries with a professional tool – a multimeter is not enough.
- Alternator check (voltage, ripple, dynamic charge).
- IBS/BMS check and battery reset/registration.
- Control of climate parameters, temperature and electrical consumption.
- Control unit error diagnosis (ECU/ABS/BCM).
- On diesel: check DPF status and regenerations .
Important: After replacing the battery, the system must always be "registered." Without this step, the Start-Stop function remains deactivated.
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High-performance car battery with 760A starting current and dimensions 278 × 176 × 190 mm . The Fiamm Ecoforce series guarantees reliable starting, greater resistance to vibration and long life, ideal for modern cars that require efficiency and stability.
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| Broken down | Solution | Average cost |
|---|---|---|
| Weak battery | EFB/AGM replacement + registration | €120–250 (EFB) / €180–350 (AGM) |
| Defective IBS/BMS | Calibration replacement or reset | €60–150 |
| Alternator/Regulator | Repair or replacement | €250–600 |
| Various sensors | Diagnosis + sensor replacement | €40–120 |
| DPF regeneration | Forced regeneration in the workshop | €70–130 |
❓ FREQUENTLY ASKED QUESTIONS
Is a Start-Stop not working a serious problem?
Usually not. In most cases, it's simply a low battery or a sensor that needs calibrating.
Can I continue driving?
Yes, but if the problem is the battery, sooner or later the starter motor may suffer.
Why does the battery seem “charged” but the Start-Stop system doesn’t work?
Because the percentage you see doesn't represent the actual capacity under load . The BMS reads more advanced parameters.
Why does it only work sometimes?
Because it only intervenes if ALL the parameters are perfect: temperature, battery, absorption, climate, sensors.
RECOMMENDED SPARE PARTS TO RESTORE START-STOP
For EFB/AGM batteries, IBS sensors, alternators, and professional testers, you can consult our complete selection:
