A warning light on your dashboard could be harmless or signal a serious fault. Understanding what each light means and knowing how to respond can save you expensive repairs — or worse, an accident.
The Universal Colour Code
Warning lights follow a standardised colour system:
- Green/Blue: information. The system is active and working normally (indicators, headlights, cruise control).
- Amber/Yellow: caution. A problem needs your attention soon. You can keep driving, but get it checked promptly.
- Red: danger. Stop as soon as safely possible. Continuing to drive may damage the vehicle or put your safety at risk.
Red Warning Lights: Stop Immediately
Engine Temperature Warning
A thermometer in water turning red means the engine is overheating. Possible causes: low coolant level, faulty thermostat, failed fan, damaged head gasket.
Immediate action: pull over, turn off the engine, wait for it to cool. Never open the radiator cap when hot. Have the car towed if the problem persists.
Oil Pressure Warning
A red oil can means oil pressure is insufficient. Without proper lubrication, the engine can destroy itself in minutes.
Immediate action: stop immediately. Check the oil level. If the level is correct and the light stays on, do not restart — have it towed.
Battery Warning
A red battery icon indicates a charging problem. The alternator is no longer charging the battery, or the battery itself is failing.
Action: you can drive to the nearest garage, but switch off all non-essential electrical consumers (radio, air conditioning). The battery will gradually drain.
Brake Warning
An exclamation mark in a red circle may indicate: handbrake on, low brake fluid, or worn brake pads. If the light stays on with the handbrake released, check the brake fluid level.
Action: if braking feels normal, get to a garage soon. If the brake pedal feels soft or sinks to the floor, stop immediately.
Amber Warning Lights: Attention Required
Check Engine Light
The check engine light is the most dreaded and most common. It illuminates when the ECU detects an anomaly in the emissions or engine management system. Causes range from a loose fuel cap to a failed oxygen sensor or catalytic converter.
Action: if the light is steady, book a garage visit within 1-2 weeks. If the light is flashing, reduce speed and get it checked urgently — an overheating catalytic converter can cause a fire.
Tip: an OBD2 reader (from around EUR 20) lets you read the exact fault code and assess severity before visiting a mechanic.
ABS Warning
The ABS light indicates a malfunction in the anti-lock braking system. Regular braking still works, but ABS won’t activate during emergency braking.
Action: get it checked within the week. Adapt your driving in the meantime — brake earlier and more gently.
Airbag Warning
An amber SRS or airbag light means at least one airbag may not deploy in a collision.
Action: get it diagnosed promptly. This is critical safety equipment.
DPF Warning (Diesel Particulate Filter)
For diesel vehicles. The particulate filter is saturated and needs regeneration.
Action: drive for 20-30 minutes at sustained RPM (motorway, 2500-3000 rpm) to trigger automatic regeneration. If the light persists, have it checked at a garage.
Most Common OBD2 Codes
When a warning light comes on, a code is stored in the ECU. Here are the most frequent:
| Code | Meaning | Severity |
|---|---|---|
| P0420 | Catalyst efficiency below threshold | Medium |
| P0300 | Random/multiple cylinder misfire | High |
| P0171 | System too lean (bank 1) | Medium |
| P0442 | Small EVAP system leak | Low |
| P0128 | Thermostat below temperature | Low |
| P0455 | Large EVAP system leak | Low |
When to Run a Full Diagnostic
- Before buying a used vehicle (to detect cleared codes)
- When a warning light comes on and stays on
- After a repair, to verify the problem is resolved
- At MOT time, if a fault is flagged
- As preventive maintenance, once a year
Use our car diagnostics tool to understand your vehicle’s fault codes and know exactly what to do before visiting the mechanic.
Sources: SAE J2012 — Diagnostic Trouble Code Definitions, ISO 15031 — Communication between vehicle and external equipment, EOBD / OBD2 European Standards (EC 715/2007).