The common ways drivers turn a small dashboard warning into a big repair bill or an MOT failure, and what to do instead.
Warning lights exist to catch problems early, when they are usually cheaper to fix. Most expensive breakdowns start with a light that was noticed and then ignored, misread or switched off. Here are ten common mistakes and what to do instead.
1. Treating every light the same
A green or blue light is information, an amber one means get it checked soon, and a red one means stop when safe. Treating a red oil-pressure light like an amber service reminder is the most costly mistake on this list. Learn the colour code and the handful of red lights in your handbook.
2. Driving on with a red oil or temperature light
Low oil pressure or overheating can wreck an engine in a short distance. "Just getting home" is often how a minor fault becomes a replacement engine. Pull over safely, switch off and call for help if needed.
3. Opening a hot coolant cap
The cooling system is under pressure when hot. Opening it can spray scalding coolant. Wait until the engine is cold before checking or topping up.
4. Ignoring a flashing engine light
A steady engine management light means book it in. A flashing one typically indicates a severe misfire, which can overheat and damage the catalytic converter. Ease off, avoid hard acceleration and get it checked promptly.
5. Clearing the code instead of fixing the fault
Cheap code readers make it easy to turn the light off. The fault is still there, the light will return, and the stored data a mechanic would use has gone. Write the code down first, and fix the cause.
6. Forgetting that some lights mean an MOT failure
Since 20 May 2018, an illuminated engine management light is a major defect. The ABS, airbag and electronic stability control warnings also lead to a fail, and on cars first used from 1 January 2012 a tyre pressure monitoring fault is checked too. Getting a light sorted before the test saves a retest.
7. Resetting the tyre pressure light without checking the tyres
The tyre pressure warning often comes on because a tyre really is losing air, sometimes from a slow puncture. Check all four tyres with a gauge, cold, against the pressures in your handbook or door pillar, and look for nails or damage before resetting the system.
8. Ignoring diesel-specific warnings
A diesel particulate filter (DPF) light usually means the filter is clogging, often on cars used mainly for short trips. Following your handbook's advice, typically a sustained run at steady speed, can let it clean itself; ignoring it can lead to a costly forced clean or replacement. An AdBlue warning means the tank needs topping up: on most cars, once the range countdown reaches zero, the engine will not restart.
9. Relying on internet guesses
Forums are full of confident answers, but symbols and fault meanings vary by make and model. Use your handbook first, and treat fault-code lookups as possibilities rather than a diagnosis. If brakes, steering or airbags are involved, get a professional check.
10. Not knowing what to do on a motorway
If a serious warning appears on a motorway, the Highway Code's advice is to leave at the next exit or services if you can. If you cannot, use an emergency area or the hard shoulder, switch on hazard lights, leave by the nearside (left) doors and wait behind the barrier if it is safe, then call for help. On a smart motorway without a hard shoulder, if you cannot get to an emergency area, stay in the car with seatbelts on and hazards on, and call 999.
A simple routine that prevents most of these
- Watch the lights at start-up: they should all go out within a few seconds.
- Check oil, coolant and tyre pressures monthly and before long trips.
- Keep the handbook in the car, or bookmark the online version.
- When a light comes on, note its colour, whether it flashes and how the car feels.
- Book amber warnings in within days, not months, and treat red ones as stop-now.
Frequently asked questions
Is it illegal to drive with a warning light on?
Not in itself, but you must keep your vehicle roadworthy. Driving with defective brakes, steering or tyres can be an offence, and a warning light may be a sign of exactly that.
Does a service reset the warning lights?
A service indicator (often a spanner symbol) is reset at a service. Fault warnings should only go out once the fault is fixed.
Sources and further reading
- GOV.UK: The Highway Code
- GOV.UK: MOT changes from 20 May 2018
- GOV.UK: MOT inspection manual for cars
- GOV.UK: Highway Code, motorways (253 to 273)
- GOV.UK: Highway Code, breakdowns and incidents (274 to 287)
Facts checked on 20 October 2026. Spotted an error? Tell us and we will correct it.