Car manufacturers agreed on a traffic-light system decades ago and have stuck to it. Whatever the shape in the middle, the colour tells you the urgency — and the colour is the same on a Fiesta, a Wrangler and a Yaris.
Red oil pressure, red coolant temperature and red brake system. Continuing to drive with any of these can turn a repair into a new engine or a crash. Everything else on this page can wait for a phone call.
The lights you will actually see
Oil pressure
Stop nowOil is not being pumped around the engine at the pressure it needs. This is not the same as “low oil” — it means the parts inside are about to run dry.
What to do: Stop as soon as it is safe and switch the engine off. Check the level when cool; if the level is fine, do not restart — call for recovery. Minutes matter.
Coolant temperature
Stop nowThe engine is overheating. Usually a leak, a failed thermostat, a dead fan or a failing water pump.
What to do: Stop, switch off, and wait at least half an hour before opening anything. Never unscrew a hot expansion tank cap. Driving on warps cylinder heads.
Brake system
Stop nowEither the handbrake is still on, or brake fluid is low, or the system has detected a fault. Low fluid usually means worn pads — or a leak.
What to do: Release the handbrake first. If it stays on, stop and check the fluid level. A soft or sinking pedal with this light means do not drive at all.
Battery / charging
Stop nowThe alternator has stopped charging. The engine is now running off the battery, and the battery is running down.
What to do: Switch off the heated screen, radio and heater and drive straight to a garage or home. Expect around 20–30 minutes before the car stops. Do not switch the engine off — it may not restart.
Airbag / SRS
Stop nowA fault in the restraint system. The airbags may not fire in a crash — or, rarely, may fire when they should not.
What to do: The car drives normally, so this gets ignored. Do not ignore it: it is an MOT failure and it is the one system you only find out about too late.
Engine management (MIL)
Attend to itThe engine control unit has logged a fault. It covers hundreds of possibilities, from a loose fuel cap to a failing sensor to a misfire.
What to do: Steady amber: drive gently and get the code read — a £20 OBD2 reader turns this into a specific answer. Flashing: a misfire that will destroy the catalytic converter. Slow down and stop.
ABS
Attend to itAnti-lock braking has switched itself off. The brakes still work normally, but the wheels can lock under heavy braking.
What to do: Drive on with more space in front of you and book it in. Usually a wheel-speed sensor. It is an MOT failure.
Tyre pressure
Attend to itOne or more tyres has lost pressure — or the system needs resetting after a tyre change, or it is simply a cold morning.
What to do: Check all four with a gauge at the pressures on the door-frame sticker. If one is repeatedly low, it has a slow puncture. It is an MOT failure if the lamp stays on.
Power steering
Attend to itAssistance has been reduced or lost. On an electric system this is usually electrical; on a hydraulic one, low fluid or a failed pump.
What to do: The car still steers, but it will take real effort at parking speed. Drive slowly to a garage rather than out onto a motorway.
Diesel particulate filter
Attend to itThe filter in a diesel exhaust is clogging with soot, usually because the car only does short cold trips.
What to do: Take it for a 20-minute run at steady speed in a lower gear than usual, so it burns the soot off. Ignore it and the filter needs a forced clean or replacement, which is expensive.
Glow plug (diesel)
Attend to itWhen it comes on with the ignition and goes out, it is normal — the plugs are preheating. Staying on or flashing means a fault.
What to do: Normal: wait for it to go out before starting. Flashing: an engine fault has been logged; get the code read.
Traction / stability control
Attend to itFlashing briefly means the system is working — you are losing grip. Staying on means it has switched off, or you switched it off.
What to do: Check you have not pressed the ESP button. If it stays lit, it is often the same wheel-speed sensor that upsets the ABS light.
Which warning lights fail the MOT
Since the MOT rules were tightened, a warning lamp being lit is a failure in its own right — the tester does not have to find the underlying fault. The lamps that fail are:
- Airbag / SRS
- ABS
- Engine management (MIL)
- Brake system warning
- Power steering
- Tyre pressure monitoring, on cars first used from 2012
Which makes a lit dashboard an expensive thing to live with: it turns a £54.85 test into a failure, a repair at somebody else’s price, and a retest. Our pre-MOT checklist has the rest.
Get the code read before anyone quotes you
An amber engine light is not a diagnosis, it is a notification. Behind it sits a specific fault code — P0420, P0300, P0171 — and reading it takes thirty seconds with a plug-in reader that costs about £20. Knowing the code before you walk into a garage changes the conversation completely: you arrive with a fault, not a symptom, and the diagnostic fee often disappears.
A code names the circuit that reported the problem, not the part that failed. “Oxygen sensor slow response” can be the sensor, or the exhaust leak upstream of it. Use it to narrow the search, not to buy parts blind.
The symbol is generic — your car's is not
Every car adds its own lamps on top of the shared set: hybrid system, adaptive cruise, lane keeping, hill descent, adblue, tow mode. The full list, with the exact wording your car uses, is in the handbook — usually a chapter called “warning lamps and indicators”.
Ford
Fiesta, Focus, Kuga, Puma and Transit handbooks, including the warning-lamp chapter and the SYNC messages that go with it.
Ford owner’s manualsJeep
Wrangler, Renegade, Compass and Cherokee handbooks — including the four-wheel-drive and hill-descent lamps that are unique to them.
Jeep owner’s manualsToyota
Yaris, Corolla, RAV4, Aygo and Prius handbooks, including the hybrid-system warnings and the READY indicator.
Toyota owner’s manualsFor other makes, the manufacturer’s UK site usually lets you download the handbook by registration or VIN. Where to find your owner’s manual covers the rest.
Common questions
The light came on and then went off by itself. Do I still need to do anything?
The fault code stays stored even after the lamp clears, so a garage can still read it. An intermittent amber light that keeps returning is usually a sensor on its way out — worth reading, not worth panicking about.
Can I just disconnect the battery to clear it?
It clears the lamp and loses the evidence. If the fault is real the light returns within a few miles, and you have thrown away the code that would have told you why. It also resets radio codes and adaptive settings on many cars.
My engine light is flashing rather than steady. Is that different?
Yes, and it is the one amber light that behaves like a red one. Flashing means an active misfire dumping unburnt fuel into the exhaust, which destroys the catalytic converter within minutes. Reduce speed, avoid heavy throttle, and stop.
Is a lit warning light an MOT failure even if the car drives fine?
For the six lamps listed above, yes — the tester records the lamp itself as the defect. It is the cheapest failure to avoid and the most annoying to discover on the day.
Sources: MOT rules on warning lamps — DVSA MOT inspection manual; MOT fee and rules — gov.uk. Symbols drawn here are the generic functional symbols; your car’s handbook is the authority for its own dashboard. Last reviewed September 2026.