DEF warning light (diesel)
Also called: diesel exhaust fluid light, AdBlue light
Diesel exhaust fluid is low or the DEF system has a fault. Ignoring it leads to a derate.
What it means
On diesel trucks and vans, the DEF light means the diesel exhaust fluid is low or the system has a problem. Emissions rules require the engine to warn you and then limit speed or power if it isn't fixed, sometimes down to 5 mph.
Green, yellow, red: three ways this can look
Match your situation to the closest example.
Keep an eye on it
The DEF level is low. Top off with fresh, sealed DEF and the light goes out.
Schedule a repair
It stays on after filling, or there's a countdown message. Book a visit; it's often a sensor, heater or doser.
Stop and get help
The truck is derated or the countdown is near zero. Stop and get help. It won't fix itself.
Common causes
- Low DEF
- Contaminated or old DEF
- DEF quality or level sensor
- DEF heater fault in cold weather
- Crystallized doser or lines
- NOx sensor fault
What to do right now
- Fill with fresh, sealed DEF; never add water
- Write down any countdown message
- If it stays on, get the codes read before the derate kicks in
Related guides
Symbols and meanings vary a little by make and model. Your owner's manual has the exact meaning for your vehicle.
Other warning lights
Questions
What does the def warning light (diesel) mean?
Diesel exhaust fluid is low or the DEF system has a fault. Ignoring it leads to a derate. On diesel trucks and vans, the DEF light means the diesel exhaust fluid is low or the system has a problem. Emissions rules require the engine to warn you and then limit speed or power if it isn't fixed, sometimes down to 5 mph.
Can I keep driving with the def warning light (diesel) on?
Green: The DEF level is low. Top off with fresh, sealed DEF and the light goes out. Yellow: It stays on after filling, or there's a countdown message. Book a visit; it's often a sensor, heater or doser. Red: The truck is derated or the countdown is near zero. Stop and get help. It won't fix itself.
What causes the def warning light (diesel)?
Common causes: low DEF; contaminated or old DEF; dEF quality or level sensor; dEF heater fault in cold weather.
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