AdBlue (DEF) warning light
The tank of AdBlue — the urea solution, called DEF (diesel exhaust fluid) in North America, that a diesel's SCR catalyst uses to turn NOx into nitrogen and water — is running low, or the SCR system has a fault. If the tank runs empty the car is designed not to restart once the engine is switched off.
Yellow / amberRed
Not defined by regulation: a maker convention
What to do
Top up with AdBlue (into the AdBlue filler, never the fuel tank) as soon as you can. Volkswagen warns first at about 1,500 miles of AdBlue range, turns the warning amber at about 1,000 miles and red when none is left; at red, stop where you can do so safely, on level ground, and top up — the engine will not restart after you switch it off until the tank has had enough. If the message names a fault rather than a low level, have the SCR system checked.
Makes differ: what this light means on your car, the other colours it may use for the symbol and what the car wants you to do are set by the manufacturer, so your car's owner's manual is the authority.
- Symbol
- Liquid container with a wavy fill line and a drop
- Colour
- Yellow / amber or Red
- Standard
- None — a maker convention, described here from 3 makers: Volkswagen, Hyundai, SEAT
Where makes differ
VW's warning starts white, turns amber, then red as the range falls; SEAT's page says that at red the engine cannot be restarted and asks for at least the minimum quantity. Hyundai's Tucson manual shows the SCR light with four escalating messages, from 'Low urea' to 'Refill urea tank or vehicle will not start', and says urea and DEF where VW says AdBlue — it is the same fluid.
Not defined by regulation: a maker convention
Neither US FMVSS 101 (Tables 1 and 2) nor UN Regulation No. 121 lists this light, so no rule fixes its symbol or its colour. What follows is what the makers cited below say; other makes, and other model years of the same make, may draw it differently.
Neither FMVSS 101 nor UN R121 gives AdBlue, urea or the SCR system a tell-tale.
Related lights a regulation does define
Other yellow / amber maker lights
All yellow / amber dashboard lights, regulated ones included
Other red maker lights
Sources
No regulation defines this light, so it is described from 3 carmakers' own manuals or owner pages, each fetched and checked on the date shown:
- Volkswagen: Volkswagen UK — Recharge AdBlue (white) warning (white, amber and red stages) — White at about 1,500 miles of AdBlue range, amber at about 1,000 miles, red when none is left; at red the car will not restart once the ignition is off until the tank has a full top-up. checked 2026-09-24
- Hyundai: Hyundai owner's manual, 2025 Tucson (general market) — Selective catalytic reduction (SCR) system (Low urea warning message) — The SCR warning light comes with four messages as the urea level falls; at the fourth the vehicle cannot be restarted once the engine is turned off. checked 2026-09-24
- SEAT: SEAT UK — Warning lights explained (AdBlue) — When red, the engine cannot be restarted because the AdBlue level is too low; stop in a suitable, safe and flat area and top up with at least the minimum quantity. checked 2026-09-24
Written in our own words; the makers' pages cover their own models only.