An instructor is doing their usual pre-lesson routine - keys, sat nav, quick glance around the car - when a warning light flickers on as the engine starts. There's a brief pause. Then the familiar internal negotiation begins.
It's probably nothing. I'll get it checked at the weekend. The car feels fine. I've got six lessons today and no time to deal with this now. I'll sort it after the test on Thursday.
Every one of those thoughts is understandable. None of them changes what the light is actually telling you.
What if the warning light is the earliest warning you're going to get?
Modern vehicle warning systems aren't decorative. They're the car's own diagnostic system flagging that a sensor reading has moved outside its expected range - oil pressure, brake fluid level, engine temperature, tyre pressure, or dozens of other monitored systems. In most cases, by the time a light appears, the underlying issue has already been developing for some time. The light isn't the start of the problem. It's the first moment you're told about it.
Plenty of faults don't change how a car feels to drive until they've become serious - low brake fluid, a slow oil leak, or a tyre pressure monitoring fault can all sit quietly in the background for weeks with no noticeable change in handling, right up until they don't.
The lights that matter most
- Brake system warning - can indicate low fluid, worn pads, or a fault in the braking system itself. Treat as urgent, every time.
- Oil pressure warning - can indicate low oil level or a lubrication fault. Continuing to drive with this light on risks serious engine damage within a short distance.
- Engine temperature warning - overheating can cause rapid and expensive engine damage if driving continues.
- Tyre pressure monitoring warning - flags a tyre that's losing pressure, which directly affects grip and stopping distance.
- ABS warning - the anti-lock braking system may not be functioning correctly, even if the main brakes still work.
- Airbag warning - indicates a fault in the restraint system, and is one of DVSA's explicit ADI test disqualifiers.
DVSA's rule leaves no room for interpretation
For the ADI part 2 test, DVSA's published requirement is unambiguous: the car must have no warning lights showing. Not "no serious warning lights." Not "nothing you'd consider urgent." None. If a light is on, the test is very likely to be cancelled on the spot, and you'll be paying to rebook on top of the disruption to your pupil's test preparation.
It's a useful standard to hold yourself to well beyond test day, too - not because every light is an emergency, but because a car that's fit to teach in should be a car with no unresolved faults, full stop.
Why "I'll deal with it later" so rarely happens on schedule
The instructors who put off a warning light rarely do it out of carelessness - it's almost always because the diary is full and there's genuinely no obvious gap. The problem is that a full diary this week is very often a full diary next week too. Without something forcing the issue back into view, "I'll sort it at the weekend" quietly becomes "I'll sort it next month," and the fault - whatever it is - has had that much longer to develop.
The daily walkaround check includes a dedicated warning light prompt, and any defect you log stays visible with reminders until you mark it resolved - so a light you noticed on Monday can't quietly disappear from memory by the time Thursday's lessons roll around.
Frequently asked questions
This article references DVSA's published vehicle requirements for the ADI part 2 test, current at the time of writing, and describes a general illustrative scenario rather than a specific reported incident. It is general information, not a diagnostic or mechanical guide - always have any warning light investigated by a qualified technician. See gov.uk for current DVSA requirements.