Will a Car Fail MOT with the Start/Stop Light On?
Modern vehicles often include start/stop technology to improve fuel efficiency and reduce emissions. However, when the start/stop warning light appears on the dashboard, many drivers wonder if their car could fail the MOT test because of it. The start/stop feature is designed to save fuel rather than directly affect safety or emissions testing. Still, some drivers get concerned when any warning light shows up during an MOT. In this guide, we explain what the start/stop system does, why the light appears, and whether it can cause an MOT failure in the UK.
What the Start/Stop System Does
The start/stop system automatically turns off the engine when the car stops, for example at traffic lights, and restarts it when the driver presses the clutch or accelerator.
Its main benefits are:
- Saving fuel in stop-start traffic
- Lowering emissions in city driving
- Reducing unnecessary engine idling
The system depends on several key components, including the battery, starter motor, alternator, and engine control unit (ECU). If one of these parts is not working properly, the start/stop function may turn itself off, and a warning light will appear on the dashboard.
What the Start/Stop Warning Light Means
The start/stop light usually looks like a circular arrow symbol with the letter A inside it.
If the light stays on or flashes, it can mean one of the following:
- Low battery voltage or weak battery health
- Faulty brake or clutch switch
- Software or ECU fault
- Incorrect engine temperature
- The system has been manually disabled by the driver
In most cases, the light does not mean there is a serious fault. Often, it simply indicates that the start/stop system is temporarily unavailable, such as when the battery is too weak or the car has not been driven much recently.
Will a Car Fail MOT with the Start/Stop Light On?
In almost all cases, no, your car will not fail its MOT if the start/stop light is on.
The DVSA (Driver and Vehicle Standards Agency) only requires MOT testers to fail vehicles with warning lights related to:
- Airbags or seatbelt systems
- Brakes or ABS
- Engine management (MIL)
- Electronic stability control (ESC)
The start/stop light does not fall under these categories because it is an energy-saving feature rather than a safety or emissions-critical system.
However, if the start/stop warning appears together with other warning lights, such as the engine management light or battery light, your car could fail the MOT because those indicate faults that affect emissions or vehicle safety.
Common Causes and UK Repair Costs
| Cause | Description | Typical UK Repair Cost |
|---|---|---|
| Weak Battery | Insufficient voltage for system | £120 – £200 |
| Faulty Brake/Clutch Switch | Incorrect signal to ECU | £60 – £150 |
| Alternator Issue | Low charging output | £200 – £400 |
| ECU or Software Fault | Communication error | £80 – £250 |
| Manually Disabled System | Driver switched it off | £0 |
What Happens During an MOT
During the MOT, the tester checks that:
- The ignition warning lights operate as designed.
- The engine management, brake, airbag, and ABS lights behave correctly.
- There are no visible safety faults or emissions failures.
Since the start/stop system is not part of the MOT’s required safety or emissions checks, its warning light is not grounds for failure. It may only be mentioned as an advisory if it stays illuminated.
How to Fix the Start/Stop Light Before MOT
- Check the battery – weak batteries often cause the light to appear.
- Drive longer trips – short journeys can prevent the system from charging fully.
- Check brake or clutch switches – replace them if faulty.
- Scan the car for fault codes – garages can do this for £50–£80.
- Update or reset ECU software – sometimes needed after a service.
If the start/stop light appears alongside other dashboard warnings, have the car diagnosed as soon as possible to rule out related issues.
Is It Safe to Drive with the Start/Stop Light On?
Yes, it is safe to drive if only the start/stop light is on. The car’s braking, steering, and safety systems will still work normally.
The light simply means that the automatic engine stop feature has been disabled. You may notice slightly higher fuel use, but there is no direct safety concern.
If the light appears alongside the battery or engine management light, however, you should have the car inspected quickly to prevent further problems.
Preventing Start/Stop Problems
- Drive longer distances regularly to maintain battery health.
- Avoid cheap replacement batteries, as start/stop cars require AGM or EFB types.
- Check your alternator output during servicing.
- Avoid leaving electrical accessories on when the engine is off.
Pre-Purchase and MOT Advice
If you are buying a used car and see the start/stop light on, it usually indicates a minor issue like a weak battery or simple electrical fault.
A CAREXAMER pre-purchase inspection can confirm whether the light is harmless or linked to a more serious problem such as an alternator or ECU fault.
For MOT purposes, the car will only fail if the light is connected to a more serious warning like the engine light or battery fault.
Summary of Start/Stop Light Causes
| Cause | MOT Impact | Average UK Repair Cost |
|---|---|---|
| Weak battery | None unless car won’t start | £120–£200 |
| Faulty switch | No MOT fail | £60–£150 |
| Alternator fault | Possible fail | £200–£400 |
| ECU issue | Advisory only | £80–£250 |
| Disabled system | No MOT fail | £0 |
Conclusion
A car will not fail its MOT if only the start/stop light is on, because the system is not considered a critical safety or emissions feature.
However, if the warning is linked to a deeper issue, such as a failing alternator or engine fault, the car could still fail under those categories.
Typical UK repair costs range from £60 for a switch replacement to £400 for an alternator fault.
If your start/stop system warning appears often, have it checked before your MOT for peace of mind. A CAREXAMER inspection can identify whether it’s a simple battery issue or something more serious.
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