Tips and advice

Engine management light: top 5 causes of amber engine warning light

Has your amber engine warning light blinked into life on your dashboard? Don’t worry, there’s no need to panic!

Engine management light: top 5 causes of amber engine warning light

The engine management light is one of the basic symbols that illuminates on your car’s dashboard. It’s usually an amber light that’s located in or around the instruments behind the steering wheel, and appears when the car’s electronics detect there is a problem with the engine.

In modern cars, a dashboard warning light is often the first indication of a problem. The warning is designed to alert you to a potential issue with your car as early as possible to prevent further damage. The amber engine warning light is the most common of these warnings and is sometimes also referred to as the ‘check engine’ light.

What is an engine management warning light?

The engine management warning light is different from the other lights on your dashboard, as it doesn’t alert you to a specific fault. Unlike the coolant temperature warning or low oil level warning lights, the engine management light (EML) can be triggered for a number of reasons.

A flashing or continuous warning light can show if the engine isn't running correctly. An EML – usually presented to depict a stylised engine block, but sometimes taking the form of a 'check engine' message – will illuminate as a prompt to seek further assistance. In the majority of cases, the car will need to be connected to diagnostic equipment to determine exactly where the fault lies.

In many cases, the light operates on a colour-coded basis. An amber EML doesn't necessarily point to an unfolding emergency – you should be able to continue on your way and seek advice on reaching your destination. A red light, though, suggests that you should pull over where it's safe to do so and arrange for your car to be recovered to a garage. Driving further with a red light could potentially cause expensive damage. Engine warning light MoT issues can stem from faults in the emissions control system, so it's worth seeking assistance sooner rather than later.

Will my car get through its MoT with the engine management light on?

In 2018, stricter MoT rules were introduced meaning that any car undergoing an MoT test with an illuminated engine management light (whether it’s a red or amber warning) would automatically fail.

Prior to taking your car for an MoT test, it’s worth checking that all warning lights go out when you start the engine; although it’s normal for the engine management light to illuminate with the ignition prior to starting, it should always go out once the car is running.

How to reset engine management light

While the documentation for some older cars provided instructions on how to check and reset engine warning lights, it's not usually possible now cars have grown more sophisticated. It's well worth paying for accurate diagnosis, though, so a proper and effective repair can be made.

Causes of engine management light

The list of possible triggers for an EML is a long one, but here are five of the most frequent causes:

Emissions system fault

Reducing engine exhaust emissions is one of the most demanding tasks a manufacturer faces when designing a new car. Nobody wants to pollute the environment excessively, but equally nobody wants a car that’s horribly stifled and can’t be driven in a normal fashion. To ensure cars are still enjoyable to drive, while preventing or reducing the emission of noxious gases, manufacturers employ a variety of control systems to manage what comes out of the exhaust.

One key component is the oxygen sensor, with many cars having more than one. This measures the amount of unburnt oxygen passing through the exhaust of the car, which can indicate whether the engine is burning too much or too little fuel. If too little oxygen leaves the exhaust, the engine will be running ‘rich’ – taking too much fuel and not enough air into its cylinders.

An EML triggered by the oxygen sensor can mean the car is running too rich or too lean, or can be caused by a fault with the sensor itself. To put this fault right may involve the reprogramming or recalibration of the engine control unit (ECU) or, sometimes, the replacement of the oxygen sensor.

Blocked diesel particulate filter or differential pressure sensor fault

Some diesel-powered cars have a specific warning light for their diesel particulate filter (DPF), whereas others don’t and rely on the EML providing a warning instead. The latter is sometimes also illuminated if the car detects a fault with the differential pressure sensor, which determines how effectively the DPF is performing.

In simple terms, this is a device that measures the ability of gas to pass along the exhaust, which includes passing through the diesel particulate filter. This is a feature fitted to diesel cars that removes carcinogenic particles from the exhaust, preventing them from polluting the atmosphere.

These particles take the form of a sooty deposit, and over time the filter becomes so heavily clogged with particles that exhaust gases struggle to pass through it. When the differential pressure sensor determines this point has almost been reached, the engine begins a process called DPF regeneration.

This involves extra fuel being burnt by the car in order to increase the temperature of the DPF. When it gets hot enough, the particulates are burnt, or oxidised, so they can pass harmlessly through the exhaust as a fine ash.

The EML (or sometimes a separate DPF light) can illuminate if this regeneration process fails to occur. Diesel cars that are only used for occasional local journeys don’t ever reach ordinary operating temperature, which is one of the conditions required before regeneration can occur. If the DPF can’t regenerate, it becomes blocked, which means the engine can’t breathe properly.

However, a faulty differential pressure sensor can also create the same symptoms. If it’s producing an incorrect reading – or none at all – the ECU doesn’t accurately know how full the car’s DPF is. It can then sometimes trigger a low-power (or limp-mode) mode to avoid the risk of damage until the fault can be investigated.

If the DPF filter has become blocked the workshop should be able to perform a ‘forced regeneration’ using diagnostic equipment to communicate directly with the engine. Its first task, though, is to make sure the differential pressure sensor is providing accurate readings.

Mass airflow sensor

The mass airflow sensor (sometimes referred to as a hot-film mass airflow sensor, air mass sensor, mass flow sensor or MAF) is vital for the efficient running of an engine. In its combustion chambers, an engine burns a mixture of fuel and air. The ECU uses data from the mass airflow sensor to calculate how much fuel to add to the combustion chambers to match the amount of air flowing into the engine.

Without this information, all it can do is guess. A modern ECU will always have a ‘safe’ set of parameters it can adopt to keep the car running if a fault arises. This is often referred to as ‘limp-home mode’ and can sometimes be triggered by inaccurate or missing data from the mass airflow sensor.

The mass airflow sensor is mounted towards the top of the engine, where it’s in the path of air that has just passed through the engine’s air filter. An incorrectly installed (or missing) air filter can damage the sensor by allowing harmful particles to make contact with it.

Alternatively, a clogged air filter not letting enough air through will trigger a warning that the engine is being forced to run ‘rich’. Whether the signal given by the mass airflow sensor is missing or outside normal parameters, the effect will be the same – an engine warning light.

Ignition system fault

Although many people refer to the starter button or key as their car’s ignition – it’s short for ‘ignition switch’ – a car’s ignition system is far more than just how it switches on and off. The ignition system is responsible for actually burning the fuel that makes an engine work.

In a petrol car, a mixture of fuel and air, in the form of vapour, are forced into the combustion chambers, where it’s rapidly burnt in a small, controlled explosion, started by a spark. This spark is provided by a spark plug, which is a component of the ignition system.

In a diesel car, the same basic process occurs, but no spark is required for combustion to take place. Instead, the mixture of diesel fuel and air is ignited by the compression of the piston acting on it. However, when you start a diesel engine on a cold day, a component called a glow plug heats the combustion chamber before the fuel/air mixture is added, making combustion easier.

Power for the glow plugs in a diesel engine and the spark plugs in a petrol is provided by the car’s ignition system. In petrol cars, a very high voltage is needed to create the spark that ignites the fuel. This is provided by the engine’s coils. Many modern cars have a coil for each cylinder, and if this coil develops a fault, the fuel in that cylinder won’t burn effectively, which not only causes the engine to run roughly or down on power, but can also cause damage to the catalytic converter in the exhaust.

While glow-plug failure is usually indicated by a separate warning light in a diesel car, problems with the spark plugs and coils of a petrol car and the wiring between them are usually covered by the EML. Faults like this are commonly referred to as a ‘misfire’ and repair often involves the replacement of one or more spark plugs, spark plug leads or coils.

Loose petrol or diesel filler cap

The last entry in our list can be one of the most frustrating, although it usually comes as something of a relief when it’s found to be the reason for your EML illuminating.

Have you ever heard a rushing noise as you open the petrol cap of your car? This is caused by a build-up of air pressure or vacuum in the fuel tank. Far from being a simple container with a supply pipe to the engine, a fuel tank is a complicated, pressurised system in itself. Fuel is pumped out of the tank towards the engine, and as the fuel is pumped out, unless air takes its place, the tank would simply crumple up like an empty drinks can.

The fuel tank has a vent that allows metered amounts of air from outside to replace the fuel as it’s pumped out of the tank. Air is allowed into the tank based on readings from a fuel tank air-pressure sensor, which is able to account for things like temperature that can also have an affect on the air pressure in the tank. If the fuel tank pressure sensor is unable to provide an accurate reading, it’ll usually trigger an engine warning light.

However, the readings from the sensor can only be reliable if the fuel tank system is 100% pressure tight, with air only permitted to enter and leave the tank through the controlled vents. One common cause of an air leak into – or out of – the petrol tank, is through a fuel filler cap that doesn’t properly seal.

This is one of the cheapest-to-fix faults that can possibly trigger an EML, with a replacement fuel filler cap often costing less than a tenner.

Read our Buying Advice guides for handy tips about buying, running and cleaning your car.

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