What do dashboard warning lights mean?

Every car runs a self-check when you switch the ignition on. A row of symbols lights up, and within a few seconds most of them go out again. The ones that stay on, or that appear later while you are driving, are the ones telling you something. This guide covers what each colour means, what the common symbols are actually asking you to do, which nine lights are tested at the MOT, and roughly what the usual repairs cost.

The colour code, and what it actually tells you

Warning lights follow a traffic light system, and it is about urgency rather than expense. Red means stop as soon as it is safe. Amber means the car will usually still drive, but something needs attention soon. Green and blue are simply confirming that a system is switched on.

The important thing to understand is that colour tells you how quickly to act, not how much it will cost. A red brake warning can turn out to be nothing more than the handbrake left partly on, while an amber engine management light can be a catalytic converter. So treat the colour as an instruction about timing, and let the garage tell you about the money.

One more distinction matters as much as the colour: whether the light is steady or flashing. That is covered further down, because with the engine management light in particular it is the difference between booking an appointment next week and pulling over now.

Red warning lights: stop as soon as it is safe

A red light means that carrying on can damage the car or leave you without a safety system. Find somewhere safe to pull in, switch the engine off, and check the obvious things before deciding whether to drive any further.

Oil pressure, the oil can symbol

This is not a top up when convenient light. It warns that oil may not be reaching the engine under pressure, and an engine can be destroyed in minutes without it. Stop, switch off, and check the level once things have cooled a little. If the level is fine and the light still comes on when you restart, do not drive it: the fault is then in the pump, the pickup or the sensor, and only one of those three is harmless. Our guide on how to check your engine oil covers doing it properly.

Engine temperature, the thermometer in liquid

Red here means the engine is overheating. Switch off and let it cool, and never open the expansion tank cap while it is hot. Driving on with a hot engine is one of the quickest ways to turn a split hose into a warped cylinder head. See why is my car overheating for what to do at the roadside, and what a head gasket repair costs for what ignoring it can lead to.

Brake system

The red brake warning covers everything from the handbrake being left partly applied to low fluid or a hydraulic fault. Check the handbrake is fully off first, because that is genuinely the most common cause. If it is off and the light stays on, treat it as a fault rather than a quirk. Checking your brake fluid takes a minute, and brake warning signs covers the noises and the feel that usually go with it.

Battery and charging

A red battery symbol while the engine is running rarely means the battery is flat. It means the car has stopped charging it, so you are running on stored power and will eventually stop, often without much warning. Head somewhere useful rather than starting a long journey. The usual causes are the alternator, its belt or the wiring. See car battery warning signs and alternator replacement costs.

Airbag and seatbelt pretensioners, the SRS light

This is the light people most often live with for months, and it is the one worth fixing first. It does not stop the car, but a lit SRS warning means the system has logged a fault and may not deploy in a crash. It is also a straight MOT failure, as the table below sets out.

Amber warning lights: keep driving carefully and book it in

Amber is the do not panic, but do not forget group. The car is normally driveable. The reason the warning exists is that these faults get worse, and more expensive, the longer they are left.

Engine management, the engine outline

The most common warning light and by far the vaguest, because one symbol covers hundreds of possible fault codes, from a loose fuel cap to a failing catalytic converter. Steady amber means book it in. Flashing means stop, because an active misfire can push unburnt fuel into the catalytic converter and destroy it. Our full guide on why the engine management light is on goes through the common causes.

ABS

Your ordinary brakes still work, but the anti-lock system may not, so an emergency stop or a wet road could take longer than you expect. The usual culprit is a wheel speed sensor, which is often one of the cheaper faults on this list. It is also an MOT failure.

Tyre pressure monitoring, TPMS

A light that comes on and stays on usually means at least one tyre is genuinely low, so check all four with a gauge, set them to the figures in the door shut or filler flap, and reset the system. If the light returns with the pressures correct, a sensor has normally failed. Those sit inside the wheel and run on a sealed battery that lasts a few years, so on an older car it is a wear item rather than a surprise. While the gauge is out, check the tread as well.

Diesel particulate filter, DPF

On a diesel, the DPF light means soot has built up and the filter needs to regenerate. Caught early this can cost nothing at all, because a sustained run at motorway speed is often enough to clear it. Ignored, it becomes a blocked filter and a real bill. See DPF cleaning costs.

Power steering

On an electric power steering system this light can mean the assistance has reduced or switched off completely. The car still steers, but it will feel much heavier, particularly when parking or manoeuvring at low speed. It is an MOT failure, and worth dealing with quickly on safety grounds regardless of the test.

Green and blue symbols are information only

Green and blue simply confirm that something is switched on: sidelights, indicators, cruise control, main beam in blue, front and rear fog lights. Nothing is wrong and nothing needs doing. The only reason to look twice is a symbol you did not switch on yourself, the classic being rear fog lights left on in clear weather, which dazzles whoever is behind you.

The common warning lights at a glance

Steady or flashing changes the answer

Two lights behave very differently depending on how they appear, and getting this the wrong way round is expensive.

A steady engine management light is a fault stored in the memory that needs reading. A flashing one means a misfire is happening right now, and every mile pushes raw fuel into the catalytic converter. Reduce speed, get off the road and switch off. A replacement catalytic converter costs far more than the recovery, as our guide on catalytic converter costs shows.

The traction or stability control light is the opposite. Flashing means the system is doing exactly what it was designed to do, usually on a wet or icy road, and it is a hint to slow down rather than a fault. It is a light that stays on steadily that means the system has switched itself off.

Which warning lights fail an MOT

This is the point where a warning light stops being a driving decision and becomes a test result. Warning lamps are testable items in their own right in the DVSA inspection manual, and most of them are recorded as Major defects. Under the categories introduced in May 2018, a Dangerous or Major defect is a fail, while a Minor defect is a pass with the defect recorded on the certificate.

Two details are worth knowing. The tester checks that each lamp lights up with the ignition and then goes out, so a bulb that never illuminates at all is recorded exactly the same way as one that stays on. Removing or disconnecting a warning light to hide a fault does not work, it fails on the same line. And a lamp showing that the passenger airbag has been deliberately switched off is not a defect, because that lamp is doing its job.

If your test is due and a light is on, get the fault read before you book. Paying for a test you already know will fail is money you do not get back, and you can check when your MOT is due free of charge. Our guides on preparing for your MOT and the most common reasons cars fail cover the rest, and what an advisory means explains the items that are noted rather than failed.

Why a light comes on and then goes off again

A light that appears for a couple of seconds when you turn the ignition on is the normal self-check, and every car does it. What matters is what happens next.

A light that comes on while you are driving and later clears by itself is an intermittent fault, and it is the one people are most tempted to ignore because the problem appears to have gone away. It has not. In almost every case the fault code stays stored in the memory even after the lamp goes out, which is exactly why a garage can still find it days later. Loose connectors, a sensor on its way out and a marginal battery all behave like this, and they all get worse. The cheap version of this repair happens while the light is still intermittent.

How a garage finds out what is actually wrong

A warning light tells you a system has complained. It does not tell you what the part is or what it will cost. That comes from plugging into the diagnostic socket and reading the stored codes, then testing whatever the code points at.

A diagnostic check commonly costs between about 40 and 80 pounds in the UK, with independent garages usually at the lower end. It is always worth asking whether the fee comes off the repair if you go ahead, because many independents do exactly that. Bear in mind that a fault code names a circuit rather than a broken part, so the code is the start of the job and not the answer to it. Our guide on what a car diagnostic check involves explains what the reading can and cannot tell you, and how to choose a good garage covers the questions to ask before you hand the keys over.

What to do when a light appears

Short version: red means stop and check now, amber means drive gently and book it in this week, flashing engine management means stop wherever it is safe. If you are unsure which you are looking at, the handbook has the exact symbol for your car, and any garage will tell you over the phone.

When you are ready to get it looked at, compare local garages and send the job to one that can read the fault properly. Tell them the colour, whether it is steady or flashing, and what the car was doing when it appeared. Those three details save diagnostic time, and diagnostic time is what you are paying for.