Which dashboard warning lights can cause an MOT fail?

Warning lights that come on and stay on are one of the more common reasons a car fails its test. The tester checks that the main warning systems work as they should, so a light that glows after the engine has started usually points to a fault that needs attention.

Not every symbol has the same weight, and the details can vary by vehicle, so use your handbook to identify what each one means. This guide covers the lights that most often matter for an MOT, what you can safely check yourself and when to stop driving.

Which warning lights will fail an MOT?

The engine management light is the most familiar. On a petrol or diesel car, if it stays lit, the vehicle can fail because it suggests a problem with the engine or emission controls. Faults in the anti-lock braking system, the airbag or seat belt restraint system and the power steering are also checked, and a warning for any of those can mean a fail. The tester will also look at the brake warning light and, where the car has one, the tyre pressure monitoring system.

• Engine management light left on
• ABS warning light on
• Airbag or restraint system warning light on
• Power steering warning light on
• Tyre pressure monitoring warning on cars fitted with it

Why is my engine warning light on and will it fail?

A steady engine light often comes from a sensor, an ignition problem, a fuel or air fault or an emissions component such as the catalytic converter. It does not always mean disaster, but it should not be ignored. A flashing light is more serious, because it can mean misfiring that may damage the catalytic converter, and the car should be driven gently, if at all, until it has been checked.

A common cause people can check safely is a loose or damaged fuel filler cap, which sometimes triggers the light. Tighten it and see whether the light clears after a few drives. If it does not, the fault codes need reading with diagnostic equipment, which is something a garage does.

Is it safe to drive with the ABS or brake light on?

If the red brake warning light is on, first check the handbrake is fully released and look at the brake fluid level when the car is parked on level ground. A low level can mean worn pads or a leak, so a garage should look at it before further driving. A red brake light that stays on is a reason to stop driving, as the brakes may not work as they should.

An amber ABS light usually means the anti-lock function is not working, although normal braking may continue. Even so, it is a safety system and the car is likely to fail with it lit, so have it checked soon.

What about airbag and power steering warning lights?

An airbag light means the system has found a fault, perhaps in a sensor, a connector under a seat or the control unit. The airbags might not deploy in a crash, so it is not something to leave. Never try to probe airbag wiring yourself.

A power steering light can mean the assistance has been reduced or lost, making the wheel heavy to turn. If the steering suddenly becomes hard to turn, slow down safely and stop where you can. Do not carry on driving with steering that feels unsafe.

What will a garage do about a warning light before my MOT?

We connect diagnostic equipment, read the stored fault codes and then carry out checks to trace the real cause, because a code points to an area rather than to a single part. We explain what we find, and you can ask for a quote before repairs begin. Repairs are only confirmed after inspection.

If a light is on, it is wise to deal with it ahead of the test rather than arriving with it lit. Automaster is a garage at Longcombe Cross near Totnes, and diagnostics and MOT testing can be arranged together.