Engine Management Light

Your car has many different warning lights on the dashboard, and they should all come on when you turn on the ignition. Once the engine’s control computer (the engine control unit or ECU) has run through its start-up procedure, they should all go off again. However, sometimes one or more light stays on, and the warning light that you are most likely to still see when all the others have gone is the engine management light (EML). This light could be displayed as a symbol which represents an engine block (highly stylised), or it could be a message that says ‘check engine’ or something similar (take a look in your handbook if you can’t identify it for your car).

Unlike most of the other warning lights that indicate specific problems, the engine management light simply means that the ECU has detected something amiss with the way the engine is running, and action is needed. There can be many different reasons why the light has come on, but the important point is that it should not be ignored. You need to take action to get whatever the issue is diagnosed and resolved before costly damage occurs to the engine, the catalytic converter or any other expensive component of your car.

The AutoMend network of vehicle maintenance, diagnosis and repair experts was established in order to provide motorists in need of EML help with access to the best possible support. Whether you are at the roadside or at home, you can contact our network members for help and advice. We have access to more than 9000 of them across the UK, so there is always someone nearby who can help.

Can I continue to drive my car?

If the EML is illuminated in red, experts advise not to continue to drive the vehicle or you risk major damage. Pull over in a safe place and arrange for the vehicle to be seen by an AutoMend mobile mechanic or transported to your local AutoMend garage for expert attention.

If the EML is amber or orange, and the car appears to be performing as normal, the advice is to continue with your journey and seek help at your destination. If the car appears to be losing power or the temperature gauge is rising, follow the advice for the red engine management light and get the car some expert attention immediately.

Can I re-set a warning light?

It is a good idea to ensure the warning light is not a total false alarm (it can happen). Pull over into a safe place and turn off the engine and ignition totally, then switch on and re-start the engine. If the warning light is still on, there is a genuine issue. If the light does not come back on at this point, it may still do in the near future, so treat it as a warning that something may be going wrong.

In some car handbooks for older models, instructions on re-setting warning lights were given. However, the lights are there to protect the vehicle, so forcing them to re-set when there is a genuine problem is pointless and you could end up with much bigger bills for repairs if you ignore the warnings.

Why has the warning light come on?

The engine management light is not a specific warning (unlike the battery or coolant temperature lights), it is simply an indicator that the ECU has detected something out of the ordinary. This makes it one of the most frustrating of problems, and often a computer diagnostic is needed in order to identify the root of the problem. However, there are several common reasons why the EML comes on. The first one may surprise you.

Loose Filler Cap

Yes, that’s right – a loose filler cap can cause the warning light to come on, so it’s a good idea to check yours before you spend money calling-out the mobile mechanics. You may have noticed that when you go to fill up with petrol or diesel, on releasing the filler cap you hear or feel pressure being released. The petrol tank is not simply a container for fuel with a pipe going to the engine, it is part of a highly complex pressurised system.

As fuel is pumped out of the tank to run the engine, an air pressure sensor within the tank allows an exact amount of air back into the tank to replace the fuel and balance the system. If this didn’t happen, the tank would begin to buckle under the pressure. The pressure sensor takes into account factors such as temperature as well as pressure, and if the filler cap is not fitting correctly, the sensor can’t give an accurate reading. The end result will be a warning light.

If the filler cap has been replaced correctly, and the system is still not 100% sealed, you may need to replace the filler cap as the seals may have been damaged.

Faulty Emissions System

Exhaust emissions are a hot topic, and every motor manufacturer is working hard to minimise them. There is always a trade-off between keeping emissions low and providing a car that performs well and is a pleasure to drive. This is usually achieved by careful control (via an exhaust system) of the noxious gases that are produced by all internal combustion engines.

One important component of the exhaust system is the oxygen sensor, and some cars have more than one of these. Their purpose is to check the exhaust gases for unburned oxygen, which is an indicator of how efficiently the engine is running. Too much or too little oxygen in the exhaust means the engine is using too little too much fuel and is therefore inefficient. If the warning light has been triggered by an oxygen sensor, the problem can be put right by recalibrating the ECU (engine control unit). This is also one of those faults where the problem could lie with the oxygen sensor itself – it may be faulty and need replacing.

Diesel Particulate Filter (DPF) blockage

In some makes of vehicle, this is a separate warning light, in others a blockage will trigger the engine management light. The DPF is a filter that removes the dangerous particles produced by the ignition of diesel fuel from the exhaust gases. There has been a lot of publicity about the harmful effects of particulate pollution, so it is essential that this filter works properly. The way the filter operates is to trap these sooty particles before they pass into the exhaust pipe, and over time, the filter becomes clogged up with the residue.

The passage of gases through the exhaust system is monitored by a device called the differential pressure sensor. When this sensor indicates the flow of air is becoming restricted by the clogged filter, a process called DPF regeneration begins. The temperature of the DPF is increased by burning extra fuel, and once it reaches a high enough temperature, the particles burn and oxidise until they turn to fine ash. This residue can then be passed through the exhaust into the environment without causing any harm.

Diesel cars which are only used for short local runs may fail to reach normal operating temperature. As DPF regeneration is triggered at full operating temperature it may fail to occur, leaving the filter clogged with particles and the engine struggling to ‘breathe’. This lack of exhaust flow will be detected by the differential pressure sensor and cause the EML to show.

Faulty Differential Pressure Sensor

The differential pressure sensor itself can also be faulty and can produce incorrect (or no) readings which can trigger the engine management light. If during diagnostics a clogged DPF is suspected, the mechanic will first check that the problem isn’t being caused by a faulty differential pressure sensor.

Mass Airflow Sensor Fault

This mass airflow sensor device comes with many names, including air mass sensor or MAF. Whatever name it goes under, it is crucial to efficient engine operation. A combustion engine has combustion chambers in which a precise mixture of air and fuel is burned. The engine control unit decides how much fuel needs to go into the combustion chambers based on air flow data from the mass airflow sensor. Pretty important information; so, if the MAF isn’t providing the right data, the ECU will trigger the engine management light, and revert to a ‘limp home mode’. This means the ECU will only allow the engine to run with a set of parameters that will keep it safe from damage until the fault has been investigated.

The MAF is located at the top of the engine, and it senses the airflow from the air filter. If the air filter is missing or installed incorrectly, the sensor can become damaged by collisions of particles. If the air filter is clogged up, the airflow will be restricted and the MAF will give an out of parameter reading. Both circumstances will trigger the EML.

Faulty Ignition System

The ignition system is not the same as the ignition switch (or key, or starter button). The ignition switch turns on the power to the car. The ignition system is the trigger which causes fuel to be burned to produce motive power.

In a petrol-engine car, a spark (from a spark plug) is used to rapidly ignite a compressed mixture of air and fuel vapour in the combustion chambers. It’s effectively a series of rapid but controlled explosions.
For a diesel-engine car, the process is basically the same, but without a spark. Compression by a piston is sufficient to cause the air and fuel mixture to ignite. On a cold day however, diesel engines require a bit of help to get the compression chamber heated prior to the arrival of the air and fuel. This heat is provided by a glow plug, and a glow plug failure can often have its own warning light. If not, a failure will trigger the EML.

The power for the glow plug and spark plugs is provided by the ignition system. For petrol cars, the voltage needed to create the spark is very high and is produced by coils, there is often one coil per cylinder. If a coil is faulty the fuel mixture won’t be combusted properly which can cause loss of power, rough running (misfire) and may even damage the catalytic converter (the most expensive single component on a car). This type of failure will trigger the EML.

How to contact an AutoMend member

With an online search function and simple no-obligation quote process, we’ve made it totally easy to get in touch with someone who can diagnose the cause of your engine management light showing. Tell us your post code, select the correct location from the choices available, pick the make of your vehicle and select ‘Diagnostics / Health Check’ from the services list. In return, we will display a list of all the businesses around your location offering the service you require, how far from you they are situated, and their contact details. Simply call them directly or ask them for online quotes with no charge and no obligation. Our members are all industry-approved experts in fault diagnosis, and they guarantee to beat main dealership prices. Give them a call today and take the mystery out of your illuminated warning light.