Will a Car Fail MOT with the Glow Plug Light On?
If you drive a diesel car, you may have seen the glow plug warning light appear on your dashboard. This often raises an important question for drivers — will your car fail its MOT test if that light stays on? The glow plug system plays a crucial role in starting diesel engines, especially in cold conditions. However, when the light stays on or starts flashing, it can sometimes signal a deeper problem with the engine management system. In this guide, you will learn what the glow plug light means, how it affects the MOT test, and what typical repair costs look like in the UK.
What the Glow Plug Light Means
The glow plug light, shaped like a small coil or spring, appears on the dashboard when the ignition is turned on. It shows that the glow plugs are heating up the cylinders to help the engine start.
Once the engine starts, the light should go off. If it stays on or flashes, it usually means there is a problem, such as:
- A faulty glow plug or relay
- Wiring or sensor issues
- A DPF (Diesel Particulate Filter) blockage
- A faulty EGR valve
- Fuel injection or turbocharger problems
In many diesel cars, the glow plug light also works as an engine management warning, similar to the check engine light in petrol cars.
Will a Car Fail MOT with the Glow Plug Light On?
In most cases, yes, a car can fail its MOT if the glow plug light is flashing or stays on due to an engine fault.
The DVSA (Driver and Vehicle Standards Agency) rules state that any illuminated engine management light (MIL) is a major defect, which means an automatic MOT failure.
Because the glow plug light often acts as part of the diesel engine management system, a flashing or permanently lit light will usually be treated as an engine or emissions fault during the MOT.
If the light comes on only before starting the engine and goes out once started, that is perfectly normal and will not cause a fail.
Common Causes and Typical UK Repair Costs
| Cause | Description | Typical UK Repair Cost |
|---|---|---|
| Faulty Glow Plug | Burnt or worn heating element | £100 – £200 |
| Faulty Relay | Glow plug relay or fuse issue | £80 – £150 |
| EGR Valve Fault | Stuck or carbon build-up | £200 – £400 |
| DPF Blockage | Exhaust filter full or clogged | £250 – £600 |
| Sensor or Wiring Fault | ECU communication issue | £100 – £300 |
| Turbocharger Fault | Boost control or pressure fault | £400 – £900 |
What Happens During the MOT
When carrying out the MOT, the tester will:
- Turn on the ignition to make sure dashboard warning lights come on.
- Start the engine to confirm the lights turn off correctly.
- Fail the car if the glow plug light or engine management light remains on or flashes.
If the light indicates a DPF fault or other emissions issue, it will also cause an automatic failure under emissions defects.
How to Fix the Glow Plug Light Before MOT
- Run a diagnostic scan to find out what is causing the warning.
- Replace any faulty glow plugs or relay, which are common and inexpensive.
- Clean or regenerate the DPF if the fault is emissions-related.
- Repair or replace EGR valves or sensors if necessary.
- Reset the system and drive the car to confirm the light stays off.
A professional diagnostic scan usually costs between £50 and £80 at most UK garages.
Is It Safe to Drive with the Glow Plug Light On?
If the light is steady, the car might still drive normally, although it may take longer to start.
If the light is flashing, it could mean a more serious fault, such as a DPF or EGR issue. Continuing to drive could damage your engine or turbo, so you should get the fault checked immediately.
Driving short distances to a garage is usually fine, but long journeys should be avoided until the problem is fixed.
Preventing Glow Plug Light Problems
- Service your car regularly and keep to the manufacturer’s schedule.
- Replace glow plugs every 60,000 to 100,000 miles as preventative maintenance.
- Use quality diesel fuel to prevent carbon build-up.
- Drive longer journeys occasionally to allow the DPF to regenerate properly.
- Check electrical connections for corrosion during servicing.
MOT and Pre-Purchase Inspection Advice
If you are buying a used diesel car and the glow plug light is flashing or staying on, it could mean there is an engine fault that will cause an MOT failure.
A CAREXAMER pre-purchase inspection can identify whether the issue is a small sensor problem or a major engine or emissions fault before you buy the vehicle.
For MOT testing, if the light is on or flashing when the engine is running, the car will fail and must be repaired before being retested.
Summary of Glow Plug Light Causes
| Problem | MOT Impact | Average UK Repair Cost |
|---|---|---|
| Faulty Glow Plug | No MOT fail if minor | £100 – £200 |
| Relay or Fuse Fault | Possible advisory | £80 – £150 |
| EGR or DPF Fault | MOT fail | £200 – £600 |
| Sensor Fault | MOT fail if light on | £100 – £300 |
| Turbo or Fuel Issue | MOT fail | £400 – £900 |
Conclusion
A car can fail its MOT if the glow plug light stays on or flashes, as this usually signals an engine management or emissions-related problem.
Typical UK repair costs range from £100 for a simple glow plug replacement to £900 for major turbo or DPF faults.
To avoid a MOT failure, identify and repair the cause early and make sure the warning light goes out before the test.
If you are unsure, a CAREXAMER inspection is an excellent way to find out whether the issue is minor or something more serious that could lead to an MOT fail.
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