In cold weather, DEF can freeze in the tank or lines. A frozen DEF will thaw and usually remain usable, but freezing can trigger warnings or delay starting. The best approach is to prevent freezing by warming and insulating the DEF system, or storing the vehicle in a heated space.
Diesel Exhaust Fluid (DEF) is a 32.5% urea/water solution used in SCR systems to reduce NOx emissions. It freezes at about 12°F (-11°C). Modern trucks have DEF heaters and tanks designed to handle freezing, but extreme conditions outside a heated environment require proactive measures from drivers and fleet operators. The following guidance offers practical steps to minimize freezing risk and maintain DEF integrity.
Keep DEF from freezing: practical steps
To reduce the chances of DEF freezing overnight, consider the following actions if you park outside in cold weather.
- Park in a heated garage or sheltered area whenever possible to keep ambient temperatures above the DEF freezing point.
- Use a DEF heater or insulated reservoir cover if your truck is equipped, and consider insulating blankets designed for DEF tanks to reduce heat loss.
- Allow the engine to warm up before starting in very cold weather; use the engine block heater as recommended by the manufacturer to raise the temperature of the engine and DEF system.
- Keep the DEF reservoir as full as practical; more mass can help with thawing and reduce the risk of ice bridging in the lines.
- Check and replace the DEF cap and seals if they show signs of moisture intrusion or damage to prevent condensation that can accelerate freezing.
In short, the above measures focus on maintaining a warmer environment for the DEF system and ensuring its seals and insulation are in good condition.
What to do if DEF freezes
If DEF freezes despite precautions, the vehicle’s DEF heater will typically thaw it once the engine has warmed up. You should:
- Drive the vehicle normally after the engine and SCR system have reached operating temperature.
- Allow extra time for the DEF to thaw and resume normal operation if you notice warnings related to DEF temperature or quality.
- Avoid trying to manually thaw the DEF in the tank with direct flame or improvised heat sources; rely on the vehicle’s built-in heating and engine warmth.
When the system is warmed, the frozen DEF should thaw and be usable again. If concerns persist, consult the vehicle’s manual or the OEM support line before continuing operation.
Storage and handling suggestions
For extended storage in cold climates, these practices help preserve DEF quality and readiness for use when temperatures rise.
- Store DEF in a clean, sealed container away from direct sunlight or heat sources; keep it in a cool, dry place when practical.
- Rotate stock so older DEF is used first, ensuring it remains within its shelf life and does not degrade in cold weather.
- Use ISO 22241-compliant DEF from reputable suppliers and avoid substitutes or adulterants.
Even with proper storage, extremely cold overnights can cause freezing; the key is to prevent freezing by maintaining temperature and ensuring the system can thaw quickly when you start driving again.
Summary
DEF freezing is a winter-weather consideration for diesel trucks. Park in a heated space when possible, use DEF heaters and insulation, follow manufacturer guidance, and avoid unapproved additives. Regular maintenance of the DEF system and proper stock management keep SCR systems functioning reliably in cold weather.