Why Has My Engine Management Light Come On?
Ellie Dyer-Brown, 4 days ago
5 min read
- Advice
- Maintenance
- why
Find out what it means if your engine management light appears on the dashboard.
Nobody wants to see that little yellow engine sign lighting up their dashboard, especially when it's flashing urgently. While it is tempting to ignore the engine management light (EML) - also known as the check engine light - for as long as possible, the reality is that you don't know how serious the fault is until you get it checked out. It might be something as simple as a loose fuel cap.
This guide covers why your engine management might be illuminated, how to fix it, and how much it’s likely to cost.
Contents:
What is the engine management light?
Types of engine management light
What to do if your engine management light comes on
What is the engine management light?
The EML is a small, engine-shaped symbol that lights up on your dashboard to signal a problem with your car. It is triggered by the onboard computer system when it detects an engine-related issue.
Types of engine management light
Engine management light and check engine light are two names for the same thing.
A yellow/orange engine management light that comes on and stays on tells you there is an issue with the engine, but it isn’t major. You can keep driving for a short distance if there are no other apparent signs of a problem, such as a lack of power.
A red or flashing engine management light is far more serious. You should stop driving immediately and contact a mechanic.
What to do if your engine management light comes on
If the yellow EML switches on, you can keep driving to your destination if it isn’t too far away. However, once you arrive, make an appointment for a mechanic to assess the problem as soon as possible. It’s a good idea to book a diagnostic test rather than trying to guess what the issue is.
If the EML is red or flashing, pull over in a safe place and contact your breakdown provider or a car recovery company. You should not continue to drive. Doing so may cause even more damage to your engine. More importantly, it puts you and your passengers in danger.
Eight reasons the engine management light comes on
Here are eight explanations for why the EML might be illuminated.
1. Oxygen sensor problems
The oxygen sensor (or O2 sensor) monitors how much unburnt oxygen is in the exhaust. It sends this information to the engine control unit (ECU) to determine the correct air-to-fuel ratio. It became mandatory to have an oxygen sensor in 1981, and vehicles produced after 1996 were required to have multiple sensors.
The most common cause of problems with the O2 sensor is contamination, which can come from burnt engine coolant, sulphur, engine additives or an excessively rich fuel mixture. It can also wear out naturally over time, requiring a replacement.
Common signs of a malfunctioning oxygen sensor include:
Decreased mileage
Check engine light
A faulty oxygen sensor can cause damage to the catalytic converter, one of the most expensive components to repair, if left unchecked.
2. Loose or broken fuel cap
Surprisingly, the engine management light will switch on if the fuel cap is not closed correctly or there is damage to the part that allows air into the fuel system. It’s easy to check and fix this issue without calling a mechanic.
3. Faulty catalytic converter
The catalytic converter is part of the exhaust system and the third most common cause of an engine warning light. It prevents dangerous fumes like carbon monoxide and other harmful pollutants from entering the air. If left untreated, it poses a risk to you and your passengers, as hazardous fumes could leak into your car's interior.
Read our ultimate guide to exhausts.
Catalytic converters can go wrong for numerous reasons, such as a build-up of carbon deposits, contamination and overheating. In addition to the engine management light, the symptoms are similar to those you experience due to a bad oxygen sensor:
Sluggish performance
Extreme heat from beneath the vehicle
Unfortunately, catalytic converters can be expensive to replace, ranging from £150 to £500.
4. Airflow problems
The mass air flow (MAF) sensor monitors the amount of air entering the engine, playing a crucial role in helping the ECU create the ideal air-fuel ratio for performance. It sits between the air filter and intake manifold. When it malfunctions, it doesn’t directly switch on the EML. Instead, it indirectly creates issues in the combustion chamber, which triggers the light on the dashboard.
The following signs indicate MAF sensor failure:
The engine struggles to start
Jerky acceleration or hesitation
Excessively rich or lean fuel mixture
Stalling
You can expect to pay between £150-200 for a MAF sensor replacement.
Similarly, a clogged air filter can trigger the engine management light because there isn’t enough oxygen in the combustion chamber. This filter is usually changed during a full service; booking one each year is an excellent way to keep your car in good condition, preventing the issues that trigger the EML.
5. Failing spark plugs
Spark plugs are responsible for initiating the combustion process within a car's engine. You can learn more about how they work and how to change them in this guide.
Eventually, they wear out and need replacing - usually every 30,000 to 50,000 miles. Your vehicle handbook should provide the correct replacement intervals; spark plugs are often changed (if required) during a full service.
You’ll know they need replacing when you notice symptoms like increased fuel consumption, rough idling and trouble starting the engine.
In diesel engines, glow plugs may be to blame for the EML. Learn how they differ from spark plugs here.
6. Dead battery
The EML may be illuminated due to an issue with the car’s battery, such as being faulty or undercharged. When your battery runs low on power, it produces low voltage, which can trigger an error code in the ECU.
Learn how to check your battery is in good working order and what to do when your battery is dead.
Modern cars are more likely to illuminate the battery warning light than the engine management light.
7. Stuck EGR valve
The exhaust gas recirculation (EGR) valve does what it says on the tin: it recirculates quantities of exhaust gas to the engine intake system to reduce Nitrogen Oxide (NOx) levels. When I had a diesel car, it quickly became one of my least favourite components due to its tendency to stick, triggering the engine management light and limp mode.
The EGR valve gets stuck due to built-up carbon deposits, often because the diesel particulate filter (DPF) hasn’t heated enough to regenerate and burn the excess soot. You can prevent this from happening by taking your car for a longer drive on a stretch of motorway once a week.
There are many symptoms that accompany a stuck valve, such as:
Rough idling
Limp mode
Check engine light
Higher emissions
Strong smell of fuel
EGR valves can be expensive to replace, costing £354.77 on average.
8. Vacuum leak
The final possibility is a vacuum leak, defined as any leak beyond the MAF sensor allowing excessive air into the system. It can be caused by a cracked intake hose or damage to one of the rubber tubes attached to the intake manifold.
There is no dashboard warning light specific to a vacuum light; instead, the engine management light illuminates, and the following symptoms may also appear.
A whooshing or whistling noise
Hard starting
Rough idle
Excessive fuel consumption
Diagnosing the cause of an illuminated engine management light can be tricky because there are many possibilities. If you’re unsure, the best course of action is to book a diagnostic test, which will provide you with the answers you need and handy advice about the type of repair required.
Frequently Asked Questions
Can I drive my car with my engine management light on?
When you start your car, the engine management light will flash on for a few seconds to show it's working. If it stays on while you're driving, this indicates a problem. Fortunately, as long as the light is steady and not flashing, you should be able to go to a nearby garage to get it checked. On the other hand, if the light is flashing, this signals a more severe problem. In this case, it's best to pull over as soon as it's safe.
Why does my engine management light keep coming on and off?
The answer to this question depends on when the light flashes on and off. It's normal for the EML to light up for a few seconds when you first start your engine, but if it flashes while you're driving, this indicates a serious fault that you should deal with immediately.
Are the engine management light and ECU light the same?
In a word, yes. Confusingly, there are a few different names you might hear people using that all refer to the same yellow light on the dash. The engine management light, engine control unit (ECU) light and check engine light are all the same.
Can the engine management light come on for no reason?
This would only happen if your car had some electrical fault causing the light to come on and stay on, which is an unlikely situation, so if you see that little light glowing on your dashboard, it's best to get your car checked by a professional to be on the safe side.
What do the other warning lights mean?
For more information, you can read our guide to what the most common car warning lights mean.
If you have any other issues with your car that you'd like to try to diagnose yourself, check out our guide on self-diagnosing car problems. The more you know about how your car works, the better.
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