Dashboard warning lights use a colour code that tells you how urgent a problem is. A red light means stop as soon as it is safe and do not carry on driving until you know what is wrong, an amber light means the car needs checking soon, and green or blue lights simply show that a feature such as main beam or cruise control is switched on. Several warning lights also matter at the MOT: a lit engine management, ABS, airbag, power steering or tyre pressure warning is a major defect and fails the test.
How the dashboard warning lights colour system works
Most manufacturers follow the same traffic-light logic, even though the symbols vary, so the colour tells you how serious a light is before you have worked out the symbol. National Highways' advice on dashboard warning lights uses the same three groups.
- Red means a possibly serious fault. Find a safe place to stop, switch off and find out what is wrong before you go any further. Do not start a journey with a red light showing.
- Amber (sometimes yellow) means a system has found a fault or needs attention. You can usually keep driving with care, but book the car in soon and do not ignore it.
- Green, blue and sometimes white lights are information only. They confirm something is switched on or working.
On a motorway, National Highways advises against stopping in a live lane unless you have no choice: if the car is still safe to drive, carry on to the next junction or services.
Your handbook is the final word on your car's symbols, as some makers use red where another uses amber. The Highway Code asks every driver to understand the warning displays on their own instrument panel and not to ignore them.
Red warning lights: stop when it is safe
Oil pressure
The oil can symbol shows that oil pressure has dropped, which is more serious than a slightly low level. Without pressure the moving parts inside the engine lose their lubrication, and serious damage can follow quickly. The AA says it is not safe to drive with this light on.
Pull over, switch off and wait a few minutes on level ground before checking the dipstick. Top up if the level is low. If the level is correct and the light stays on, the cause may be the oil pump or a blocked filter, so call for recovery.
Engine temperature or coolant
A thermometer in wavy lines means the engine is running too hot, or on some cars that the coolant level is low. Common causes are a coolant leak, a split hose, a failed water pump or a cooling fan that has stopped working.
Stop and switch off. Never open the coolant cap while the engine is hot, as the pressurised system can spray scalding coolant. Once it has cooled, check the level, top up and look for leaks. If the light comes back, get the car checked.
Brake system
The red brake light is usually a circle with an exclamation mark or the word BRAKE. The first thing to check is that the handbrake or electronic parking brake is fully off. If it is and the light is still on, the brake fluid may be low or there may be a fault in the hydraulic system.
The RAC advises pulling over safely, keeping your speed low and avoiding sudden braking, then calling for recovery. A soft or sinking pedal makes this more urgent. If the red brake light and the ABS light come on together, treat it as a brake fault and stop.
Battery or charging
The battery symbol does not usually mean the battery is old. When it lights up with the engine running, it means the battery is not being charged, often because of the alternator, its drive belt or the wiring. The car is running on whatever charge the battery has left, and it can lose power without much warning.
Switch off electrical extras such as heated seats and the rear screen heater. The AA's advice is to stop safely, turn the engine off and not restart it, as that can make the problem worse.
Amber warning lights: get it checked soon
Engine management light
The engine outline, often called the check engine light, comes on when the engine control unit records a fault, many of them emissions-related: a sensor, the ignition system, the EGR valve or a blocked particulate filter.
A steady amber light usually means you can finish your journey if the car drives normally, then get the fault codes read as soon as you can. A flashing light is more serious. The AA's engine management light guide says it can point to a problem affecting the catalytic converter, often caused by a misfire. Ease off, avoid hard acceleration and high revs, and stop when safe.
At the MOT, an engine warning light that is on or not working is a major defect. Under section 8.2 of the MOT inspection manual, this applies to most petrol cars first used from 1 July 2003 and diesels first used from 1 July 2008.
ABS
ABS in a circle means the anti-lock system has switched itself off after finding a fault, often a wheel speed sensor. Your normal brakes should still work, but the wheels may lock under hard braking in the wet, so leave more space and get it checked. An ABS warning that shows a fault, or that does not work at all, is a major MOT defect under section 1 of the manual.
Airbag (SRS)
A seated figure with a circle in front of it means the supplementary restraint system has found a fault, which could involve an airbag, a seatbelt pretensioner or the wiring under a seat. Many cars show this light in amber, though some brands use red. The RAC warns that with this fault the airbag may not go off in a crash. The car will still drive, but book it in quickly. An SRS warning light showing a malfunction is a major MOT defect, and so is a seatbelt pretensioner that is missing or has already been deployed.
Tyre pressure (TPMS)
The tyre pressure light looks like a horseshoe-shaped tyre with an exclamation mark inside. At least one tyre has lost pressure, from a slow puncture, a damaged valve or a cold snap. Inflate all four tyres to the pressures in your handbook, then reset the system if needed. If the light returns soon after, assume a puncture and get the tyre looked at.
On passenger cars first used from 1 January 2012, a tyre pressure monitoring system that is not working is a major MOT defect, according to section 5 of the manual.
Diesel particulate filter (DPF)
A diesel's particulate filter traps soot and burns it off when the exhaust is hot enough. Short, slow journeys stop that happening, so the filter clogs and the light comes on. The AA suggests driving for around 10 minutes at over 40mph, when it is safe and legal, to give the filter a chance to clear. If the light stays on after that, contact a garage or your breakdown provider. Ignoring it can lead to a costly forced clean or a new filter. Removing or tampering with a DPF is a major MOT defect.
Glow plug
The glow plug light, a coiled wire symbol, only appears on diesels. It is normal for it to come on when you turn the key: it shows the plugs are warming the cylinders, and you should wait for it to go out before starting the engine. If it stays on, comes back while you are driving or flashes, many manufacturers use it to flag an engine fault, so check your handbook and get the codes read.
Green and blue information lights
These lights do not mean anything is wrong. A blue headlight symbol with straight lines shows that main beam is on. Dip it for oncoming traffic and when you are following another car, so you do not dazzle other drivers. A green speedometer symbol shows that cruise control is active or set.
Other lights worth knowing
A steering wheel symbol warns of a power steering fault. The steering may suddenly become much heavier, so slow down and take care at junctions. The RAC suggests switching off, waiting 30 seconds and restarting, which sometimes clears an electrical glitch. An electric power steering warning showing a fault is a major MOT defect under section 2 of the manual.
The stability control light (a car with skid marks) flashing on a slippery road just means the system is working. If it stays on, it has been switched off or has a fault, and a stability control malfunction warning is also a major MOT defect.
A lit or faulty brake fluid warning is different: the tester records a minor defect, which does not fail the car on its own.
A spanner or service symbol is a reminder that the car is due a service. For the routine checks that keep most warning lights from appearing at all, see our car maintenance checklist.
Intermittent warning lights
A light that comes on for one journey and then disappears is easy to dismiss, but the engine control unit usually stores a fault code after the light goes out, so a garage or code reader can still see what triggered it. Fixing a small fault early is usually cheaper than dealing with the damage later.
Warning lights on a used car you are viewing
Every warning light should come on briefly when you turn the ignition on, as a self-test, and then go out once the engine starts. The Highway Code describes the same check. When you view a car, turn the key to the ignition position before starting and watch the dashboard.
- If the engine management, airbag, ABS or other warning light does not light up at all, ask why. Removing a bulb, unplugging a lamp or coding it out is a known way to hide a fault, and an MOT tester will fail a warning lamp that is not working.
- If a light stays on after the engine starts, the seller may say it is "just a sensor". It might be, but you have no way of knowing until the codes have been read.
Then look at the car's past. The free MOT history on GOV.UK lists every failure and advisory since 2005, and repeated items such as "engine MIL indicating a malfunction" or an SRS fault show a problem that keeps coming back. Our guide to checking MOT history before you buy explains how to read these records, and the most common MOT failures gives context on which faults are routine and which are expensive. You can also run the registration through a free car check on CheckVehicle to see the MOT history and DVLA details in one place.
Before you pay
- Do the ignition test yourself and make sure every warning light comes on, then goes out.
- Ask for a diagnostic scan if any light stays on or behaves oddly.
- Agree in writing who will pay to fix a warning light that is on at the time of sale, or factor the repair into your offer.
- Work through the rest of our used car buying checks before handing over money.
Frequently asked questions
Is it safe to drive with the engine management light on?
If the light is steady amber and the car drives normally, you can usually finish your journey carefully and book a diagnostic check soon. If it is flashing, or the car has lost power, stop when safe, as a misfire can damage the catalytic converter. A red engine light means stop and call for help.
Will a warning light fail my MOT?
Several will. An engine management, ABS, airbag, electronic power steering, stability control or tyre pressure warning showing a fault is a major defect and fails the test. A lit brake fluid warning is recorded as a minor defect, so it does not fail the car by itself.
Why do all the warning lights come on when I start the car?
It is a self-test showing the lamps and systems work. They should go out once the engine is running; if one stays on, or never lights up, get it checked.
Can I clear a warning light with a code reader?
You can clear the fault code, but the light will come back if the fault is still there. Clearing it just before an MOT or a sale hides the problem for a while and does not fix it. When you are buying, a recently cleared code is something to ask about.
What is the difference between a red and amber airbag light?
It depends on the manufacturer: some show the airbag warning in amber, others in red. Either way it means the restraint system has found a fault and the airbags may not work in a crash, so get it checked promptly.
