Diesel exhaust fluid (DEF) warning light on

Symptoms

• DEF level warning light or gauge showing low
• Countdown message on some vehicles showing miles until reduced power
• No change in performance yet if caught early
• Possible reduced power or speed limit if ignored too long

Likely causes

• CommonLow DEF fluid levelDEF is consumed gradually and needs periodic refilling separate from diesel fuel.
• PossibleDEF quality or contamination issueExpired or contaminated DEF can trigger a fault even with adequate fluid level.
• PossibleFailing DEF level sensorA bad sensor can show low or empty even when fluid is present.
• Less commonSCR system fault (injector, heater, or pump)A malfunction in the emissions system that uses DEF can trigger warnings independent of fluid level.

What to do

1. Check the DEF level using the gauge or dipstick if your vehicle has one.
2. Add DEF fluid from a sealed container as soon as convenient; it's sold at most auto parts stores and truck stops.
3. Use only fluid meeting the ISO 22241 standard and avoid contaminating the fill port with dirt or other fluids.
4. Don't ignore the warning, since many vehicles will limit speed or prevent restarting once DEF is fully empty.
5. If the light stays on after refilling, have the DEF sensor and system checked by a shop.
6. Keep an eye on DEF consumption over time so you can refill proactively before the warning appears.

Tools and parts

When to call a pro

See a shop if the warning persists after a proper refill, since that points to a sensor or system fault rather than just low fluid.

Safety