Dashboard warning lights to watch for when buying
Which red and amber warning lights mean stop or check soon, which ones fail the MOT, and how to spot a seller hiding a fault by disabling a bulb.
Updated
When you view a used car, the warning lights tell you two things. First, what is wrong now: a light that stays on after the engine starts is a fault to explain before you buy. Second, whether anyone has tried to hide a fault: every warning light should come on when you turn the ignition on and then go out. A light that never comes on at all may have been disabled. Several warning lights are also part of the MOT, so a car with a lit airbag, ABS or engine management light can fail.
The basic rule: on at ignition, then off
The Highway Code says that when you turn the ignition key, warning lights will come on but should go out when the engine starts, except the handbrake warning light. If they do not, or if they come on while you are driving, it says to stop and investigate, because you could have a serious fault.
That gives you a simple test at a viewing:
- Sit in the driver's seat with the engine off.
- Turn the ignition to the position before starting, or press the start button without your foot on the brake or clutch, depending on the car.
- Watch the dashboard. The warning lights should all light up for a moment. That is the bulb check.
- Start the engine. The lights should go out within a few seconds, apart from the parking brake light if the parking brake is on.
Some cars use a screen instead of separate bulbs and show a message as well as a symbol. The idea is the same. Check the handbook if you are not sure what a symbol means.
Red lights: stop and find out
Red usually means a problem that needs attention straight away. Colours and symbols vary between cars, so treat this as a general guide.
| Light | What it usually means | At a viewing |
|---|---|---|
| Oil pressure | The engine is not getting enough oil pressure | Do not drive the car until it is explained |
| Brake system | Low brake fluid, a brake fault, or the parking brake on | If it stays on with the parking brake off, do not drive it |
| Coolant temperature | The engine is overheating | Stop. See our head gasket guide |
| Charging or battery | The battery is not being charged | The car may stop when the battery runs flat |
The Highway Code points out that a charge warning light can mean the battery is not charging, and that low brake fluid may result in brake failure.
Amber lights: check soon
Amber or yellow usually means a fault to have checked soon. On a car you are thinking of buying, "soon" means before you pay.
- Engine management light. A fault in the engine or emissions system. It has many possible causes, from a loose fuel cap to a serious engine problem. A garage can read the stored fault codes.
- ABS. A fault in the anti-lock braking system. The normal brakes may still work, but the anti-lock function may not.
- Airbag or SRS. A fault in the airbags or seat belt pretensioners. They may not work in a crash.
- Stability control (ESC). A fault in the system that helps stop the car skidding.
- Tyre pressure monitoring (TPMS). Low tyre pressure, or a fault in the monitoring system.
- DPF. On diesels, the particulate filter is filling up. See our guide to DPF problems.
- Glow plug. On diesels, it normally lights briefly before starting. If it flashes or stays on, there may be a fault.
- Power steering. A fault in the electric power steering.
Which warning lights fail the MOT
The DVSA inspection manual includes several warning lamps. These are the main ones, as the manual describes them at the time of writing.
| Warning lamp | Defect in the manual | Category |
|---|---|---|
| ABS | Warning device not working, or showing a system malfunction | Major |
| Airbag (SRS) | Malfunction indicator lamp indicates a system malfunction | Major |
| Stability control (ESC) | Malfunction indicator lamp indicates a system malfunction | Major |
| Engine management | Lamp inoperative or indicating a malfunction | Major |
| Electric power steering | Malfunction indicator lamp indicating a system malfunction | Major |
| Electronic parking brake | Malfunction indicator lamp indicates a malfunction | Major |
| Tyre pressure monitoring | System malfunctioning or obviously not working | Major |
| Brake fluid | Warning light illuminated or defective | Minor |
A major defect fails the test. A minor defect is recorded but the car passes. Our guide to MOT advisories and failures explains the categories.
A few details matter:
- Engine management is checked on most petrol cars first used from 1 July 2003 and diesel cars first used from 1 July 2008.
- Tyre pressure monitoring is checked on passenger cars first used on or after 1 January 2012. The manual notes that a TPMS light can stay on simply because a tyre was low, which is not automatically a malfunction.
- A passenger airbag "off" light is not a defect when it is only showing that the passenger airbag has been switched off.
- Brake pad warning lights were added to the MOT checks when the test changed on 20 May 2018, but the current manual says an illuminated brake wear indicator is not on its own a reason for failure. Pads worn down to the wear indicator are a major defect.
How sellers hide warning lights
A warning light that is on is easy to see. A warning light that has been disabled is not. Sellers have been known to remove a bulb, disconnect a lamp or clear stored fault codes just before a viewing. A cleared fault can come back after a few miles.
That is why the bulb check matters. Look for each light during the ignition-on moment. If a light you would expect to see, such as the airbag or ABS light, never comes on at all, ask why. A missing bulb check is a warning sign in its own right.
Other things to watch for:
- A car that is already running when you arrive. You miss the bulb check. Ask the seller to switch it off, wait a minute, and do it again with you watching.
- Lights that come on after a long drive. Some faults only show once the engine is warm or the car has done a few miles. Take a proper test drive, and use our test drive checklist.
- A recently disconnected battery. This can reset some warnings temporarily.
The MOT also checks some of these lamps for operation, so a disabled light can fail too. But you should not rely on the next MOT to find it.
What to do if a light is on
- Ask the seller what it is and what has been done about it. Ask for invoices.
- Do not accept "it's just a sensor" without evidence. It may be, but a sensor fault can still fail an MOT, and the repair can cost real money.
- Ask for a diagnostic check. A garage can read the fault codes. Some sellers will agree to this at a garage of your choice.
- Price it in or walk away. If the seller will not explain or fix it, factor the unknown cost into your offer, or look at another car.
If you are not sure, see the car and ask a mechanic.
Frequently asked questions
Which dashboard warning lights are most serious?
Red lights, especially oil pressure, brake system and coolant temperature. If one stays on after the engine starts, do not drive the car until the cause is known.
Will a car fail its MOT with the engine management light on?
On most petrol cars first used from 1 July 2003 and diesels first used from 1 July 2008, yes. The DVSA manual makes a lamp that shows a malfunction a major defect.
Can a seller hide a warning light?
Yes, by removing a bulb, disconnecting a lamp or clearing fault codes. Watch for every light to come on when the ignition is switched on, then go out.
Is a tyre pressure warning light an MOT fail?
A malfunctioning or obviously not working TPMS is a major defect on cars first used on or after 1 January 2012. A light that is on only because a tyre is low is not automatically a malfunction.