AdBlue / DEF Warning
Diesel exhaust fluid (AdBlue/DEF) is low. Most diesels will refuse to restart once it runs out.
What it means
Modern diesels inject DEF into the exhaust to neutralize NOx emissions. The warning usually starts around 1,500 miles of range remaining with a countdown. Regulations require the engine to be immobilized when the tank is empty — many cars will not restart after you switch off.
Can you keep driving?
Yes, until the countdown reaches zero — but top up at the next opportunity. Buy sealed DEF (ISO 22241) from any parts store or many fuel stations; the filler usually has a blue cap.
What to do now
- 1
Top up with fresh, sealed DEF — do not dilute it or substitute anything else.
- 2
Use the correct filler (blue cap) — DEF in the fuel tank is catastrophic.
- 3
If the warning persists after a proper fill, have the DEF quality/level sensors checked.
Likely causes
- • DEF tank low
- • Contaminated or diluted DEF
- • Faulty level or quality sensor
- • Frozen DEF in extreme cold (the heater usually resolves this)
Typical fix cost
$10–$20 for 2.5 gallons of DEF; $200–$800 for sensor or tank heater faults
Ranges reflect mainstream US parts-and-labor pricing; your vehicle and region will vary.