Dashboard warning lights: what to do when one comes on

Symbols and colours vary between makers, and DVSA’s national standard for driving cars says it’s vital to use the handbook to find out how a car’s instruments work. As a rough guide:

• Red usually means a serious problem or a safety warning, and often that you should stop when it’s safe.
• Amber or yellow usually means something needs checking, or a system has switched itself off. Get it looked at soon.
• Green or blue usually just shows that something is on, such as the headlights or cruise control.

Many lights come on briefly as a check when you switch on, then go out once the engine starts. The Highway Code says that if one doesn’t go out, or comes on while you’re driving, you should stop and investigate the problem, as you could have a serious fault. Some lights flash to show a different or more urgent problem. A spanner symbol or a ‘service due’ message is usually just a reminder: see our guide to interim, full and major services.

The Highway Code also says to make sure you understand every warning display on your instrument panel. Handbooks often say to stop as soon as it’s safe for red warnings like these:

• Oil pressure (usually an oil can): the engine may not be getting enough oil, and running it like that can damage it.
• Engine temperature (usually a thermometer, or a gauge in the red): a sign the engine is running too hot. Let it cool before you open anything under the bonnet.
• Brake system (often an exclamation mark in a circle): on many cars it also shows while the parking brake is on, so check that first. If it stays on, it can mean low brake fluid or another brake fault, and the Highway Code warns that low brake fluid may result in brake failure and a crash.

If you need to stop, the Highway Code says to try to do it in a place of relative safety, such as somewhere designed for parking, or a lay-by. On a motorway, leave at the next exit or pull into a service area if you can.

If the handbook says not to drive on, don’t. At the roadside, call your breakdown provider; if the car is parked somewhere safe, you can post a mobile mechanic callout. Garages choose whether to quote on a callout as on any job, so there’s no set response time, and it isn’t a breakdown or emergency service.

Here’s what these usually indicate. Whether it’s safe to carry on, and how soon to have the car checked, is in your handbook, so check it once you’ve stopped.

• Engine management (an engine outline): usually means the engine’s control system has logged a fault. On many cars a flashing light is more urgent than a steady one.
• ABS: the anti-lock brakes may have a fault. Check the handbook for whether the ordinary brakes are affected.
• Airbag or SRS: the airbags or seat belt pretensioners may not work as designed in a crash.
• Stability control, also called ESC, ESP, VDC or DSC: flashing usually means it’s working, on a slippery road for example, while a light that stays on can mean a fault or that the system is switched off.
• Tyre pressure (TPMS): a tyre may be low, or the system may have a fault. The Highway Code says to check pressures weekly, when the tyres are cold, against the figures in the handbook.
• Battery or charging (often red): the battery may not be charging, and the car can stop once it runs down, so get it checked promptly.
• Power steering: the steering may feel heavier than usual.
• Glow plug, on diesels (usually a coil): it normally shows before you start, then goes out. If it flashes or stays on, check the handbook.
• DPF, on diesels: usually means the diesel particulate filter is getting blocked with soot. GOV.UK’s guidance on DPFs explains how the filter traps soot and burns it off.
• AdBlue, on some diesels: usually means the fluid is low or its system has a fault. If it runs out, some cars won’t restart until it’s refilled, and they warn you first.

The MOT tester checks several warning lights, and one showing a fault can fail the test even when the car drives normally. This is how the MOT inspection manual grades them. A major defect fails the test; a minor one is listed on the result but doesn’t fail it.

• Engine warning light: the tester switches on the ignition to check it lights up, then checks it goes out, which on some cars happens only once the engine is running. One that doesn’t work, or shows a fault, is a major defect on petrol cars first used on or after 1 July 2003, and on diesel, gas and bi-fuel cars first used on or after 1 July 2008, hybrids included.
• ABS: a warning light that doesn’t work, or shows a system fault, is a major defect.
• Electronic parking brake, electric power steering, airbags (SRS) and stability control (ESC): a warning showing a system fault is a major defect. A light that only shows the passenger airbag is switched off isn’t a defect. Because stability control lights vary, the manual says the tester may need to check the handbook.
• Tyre pressure (TPMS): on passenger cars first used on or after 1 January 2012, a system that has a fault or obviously isn’t working is a major defect. A light left on by an earlier loss of pressure doesn’t automatically mean a fault; a fault may show as a light that flashes several times, then stays on.
• Brake fluid warning light: on or not working is a minor defect. The fluid level itself is checked separately.
• Brake pad wear warning: a lit wear warning isn’t a reason to fail on its own, but pads worn down to the wear indicator are a major defect.

If a warning light is on and the MOT is coming up, get it looked at first. There’s more in our guides to what an MOT checks and MOT results.

On Matchanic, the Diagnostic list has jobs for common lights, such as ‘Engine management light’, ‘ABS warning light’ and ‘Tyre pressure (TPMS) warning light’. Choose the one that matches, or ‘Other diagnostic’, then add under ‘Anything else’:

• What the symbol looks like, its colour, and any message with it.
• Whether it’s steady or flashing, and whether it goes out after a restart.
• When it came on: from cold, at speed, after filling up or after recent work.
• What the car is doing: a noise, a smell, smoke, less power or heavier steering.
• Any fault code you’ve already been given.

A garage usually starts by reading the fault codes the car has stored, so leave them in place rather than clearing the light. Garages on Matchanic can see your job and choose whether to quote. There’s more on the Diagnostics and warning lights page.