Engine management lights: all you need to know

What are engine management lights?

The engine management light (EML) warns the driver that something has happened to do with the engine that may affect the exhaust emissions. That is all that motorhome engine management lights do.

The lamp is the visual part of the EOBD system (European On-Board Diagnostics), and the system includes a socket through which suitable diagnostic devices can retrieve fault codes that will give clues, if not definitive answers, of what is going wrong.

The lamp is orange in colour and features the outline of an engine. It may be on or it may flash and there may be warning messages displayed in the instrument panel that give guidance such as ‘glow plugs’ or ‘stop engine’. The latter is an instruction.

It should illuminate when the engine is started and should disappear within a couple of seconds. It should not be confused with a symbol depicting a car with a spanner through it, which refers to the need to perform a service.

How serious is it?

The possible reasons for the EML becoming illuminated or staying illuminated after starting up are far too numerous, but the bottom line is that, while the light is on, the vehicle would fail an MoT test and, if it could not pass an MoT, it is not roadworthy.

If it is not roadworthy, it cannot be used on the road. Most vehicle handbooks will advise you to contact a dealership as soon as possible, but the lamp is not always a catastrophe in the making and may be nothing to worry about at all.

If the fault was a one-off and the next time the engine is started, a problem is not present, then the EML may be able to reset itself, but will still have recorded a ‘stored’ code for later examination. If the fault is still active, the code would be stored as ‘present’ and the EML will remain illuminated.

The vehicle can protect itself from potential damage by allowing the engine to run in a ‘limp home mode’ if it deems this necessary, but this mode of operation will limit power, and will normally increase fuel consumption and emissions because it is no longer ‘optimised’.

Is it safe to drive with?

Generally speaking, in the event of an EML becoming illuminated while driving and without any alarmist messages being displayed (on vehicles with this facility), the best thing to do would be to stop as soon as it is safe to do so and turn off the engine.

First of all, if you are able to, check under the engine for any leaking oils or fluids and then lift the bonnet and check that the coolant level is within limits (without removing any caps). After a couple of minutes, check that the oil level is reasonable and that there are no strong smells or leaks of fuel in the engine bay.

If all seems well, you should get back inside and start the engine again. Quite often there will be no warning light and you can make a mental note to ask the garage to plug into the EOBD to see what it was next time it is being serviced.

If the restart does not remove the EML, particularly in the case of vehicles fitted with diesel particulate filters (see below), I would recommend that you drive only a short distance if you are nearly where you are going (maybe 50 to 60 miles maximum).

If you have too far to drive, you should use a mobile service to examine the vehicle and observe its advice regarding onward travel or remedial action.

There are many possible reasons for an EML to light up, and some of them are fairly innocuous. Some of them are fairly minor in their nature and may not cause harm to the engine, even in the medium term.

The risk of clogging up a DPF and the costs that come with that, or not knowing about a low oil pressure situation until it is too late, are too significant to ignore.

Add to this that a vehicle that is limping around with an EML on the dashboard is not compliant with being in a roadworthy condition and, if discovered, could be removed from the road, and the case for not ignoring the warning could not be clearer.

Will my car pass an MOT with them on?

Recent changes to MoT procedures mean that an EML, an ABS or other brake warning, a tyre pressure monitoring lamp, traction control and power steering warnings will fail the test.

If a vehicle is driven with any of these warning lamps on, it is not roadworthy – whether it has a current MoT certificate or not.

Common causes for the engine management light to turn on

Under the bonnet of a modern diesel engine, there are dozens of devices that measure, control, adjust and monitor the process of adding fuel to air and burning it as efficiently as possible.

All of these devices report to the engine’s electronic control unit (ECU) and, if the ECU receives a signal voltage or measurement that is not as expected, a fault code will be generated. If that fault is considered significant it will trigger the EML.

The EML may have been lit due to a minor voltage fluctuation or a poor connection between a cable and a device. Although it is not supposed to be possible, I have even seen an EML that was caused by low air-conditioning pressure!

However, one of the most common causes is with diesel particulate filters (DPF). In the past, a sign that a diesel engine was not running at its best would be a little extra smoke or soot coming from the exhaust and we would have known to get the vehicle looked at as soon as we could and could monitor the severity of it ourselves.

Or ignore it completely as many owners did until it was time for an MoT test. Those days are gone.

Even a fairly minor fault with the fuel injection system can cause more smoke and/or soot to be produced.

All of the smoke and soot particles produced by a vehicle so equipped will be captured by the DPF and, if the rate of accumulation is much greater than the DPF was designed to cope with, this will lead to ‘clogging’ and, if this is not dealt with soon, the clogging can become so excessive that the vehicle cannot clean the DPF as it normally would and, in extreme cases, there may need to be major repairs or replacement of the DPF.

For this reason more than any other, a vehicle with a DPF will need to have any warning messages or EML looked into as a matter of urgency.

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How to resolve the issue

The garage will almost certainly have to plug in a diagnostic device in order to retrieve fault codes and determine the nature of the problem. Generally, only franchised dealers will have the manufacturer-specific devices to do this, but independents may also have these or multipurpose units to deal with several brands of vehicle.

Such devices can clear the fault codes and reset the engine management light, but if the fault still exists, the light will come back, so it is important to get it checked out.

Not all of these units will be as good as the manufacturer kit and some offer very limited capabilities, so it is essential that you check that the garage has equipment that is capable of digging deep into the specific systems that you have and, more importantly, that it has experience with your vehicle in particular.

The reason for this is that each manufacturer deals with specific fault codes and descriptions in its own particular way and the meanings can be quite different.

For example, an experienced technician looking at a Ford vehicle and seeing a description of a fault with an airflow meter will know that the fault is most likely going to be the airflow meter and can confirm it by taking live measurements, while a Citroën specialist might know that the most likely thing that causes an airflow meter fault is actually a restricted air pipe.

The right knowledge and tools can save time and money by avoiding replacing parts that are not at fault.

It is a fact that some fault codes are generated because of unexpected measurements and these can arise from, for instance, the wrong quantities of the incorrect gases in the exhaust system.

Some technicians might suspect that the fault originated from a sensor in the exhaust piping and replace it while a more experienced technician would realise that the problem was caused further upstream in the combustion process and that the measurements at the sensor, while not as expected, were correct and caused by something else.

Experience of these sorts of issues is vital to a successful and cost-effective outcome.

Other motorhome engine warning lights

There are many other warning lamps and some of these are generic and others are quite specific to a manufacturer.

Generally, they are going to be orange or red. A red warning indicates that either you or the vehicle are in danger. An airbag lamp, a brake warning or seatbelt lamp should be regarded as hazardous to you, while a red battery symbol, an oil can or temperature symbol tells you that the vehicle is in danger of being harmed. All red symbols are urgent and require immediate attention. You or the vehicle could be compromised by ignoring these.

An orange lamp may warn of a malfunction with glow plugs, tyre pressures, anti-lock brakes, traction control, a DPF (operation or fault) or low fuel level to name a few. These are advisories that continued use of the vehicle may cause unexpected effects or reduced levels of safety from those that you are accustomed to.

They should be attended to as soon as possible and limited driving with care is usually permissible, but there is no substitute for checking your owner’s handbook for manufacturer-specific details and guidelines.

Increasingly common is the use of a temperature symbol that starts off blue in colour when the engine is cold and then disappears once up to operating temperature. It will appear again as a red version of the same symbol if the engine becomes too hot and is overheating.

Final thoughts

It’s important to stress that the engine management light is just an indicator that something could be wrong somewhere in the system.

The same goes for fault codes – a good technician should be able to use their knowledge to trace the source of the problem rather than rely on the very basic information (which can be misleading) that the diagnostic device outputs.

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