Urgency: Moderate
DEF / AdBlue Warning Light
Investigate soon. Driving short distances is generally okay, but book a diagnostic check.
What the DEF / AdBlue Warning Light Means
The DEF/AdBlue light on a diesel your car warns that the diesel exhaust fluid is low. Ignore it and the countdown will eventually prevent the engine from restarting — a legal emissions safeguard, not a mechanical fault.
Common Causes
- Low diesel exhaust fluid level (normal)
- DEF quality/contamination
- Faulty DEF level or quality sensor
- SCR system fault (P204F)
- Crystallised DEF injector
How to Fix It
- Top up with the correct AdBlue/DEF fluid
- Add enough to clear the low threshold (usually several litres)
- Wait for the system to re-read the level
- If it will not clear, scan the SCR system
- Have the DEF sensor or injector checked if faults persist
Related DTC Codes
| Code | Meaning |
|---|---|
P204F | Reductant System Performance The selective catalytic reduction (AdBlue/DEF) system is underperforming. |