A04-33161-005 | Genuine Freightliner® Diesel Exhaust Fluid (DEF) Tank 23 Gal

To reduce harmful exhaust emissions
The DEF tank supplies Diesel Exhaust Fluid that helps convert nitrogen oxides (NOx) in diesel exhaust into harmless nitrogen and water vapor.

To meet government emission regulations
DEF allows diesel vehicles to comply with strict environmental laws (EPA, Euro standards) and remain legal to operate.

To maintain engine performance and efficiency
By cleaning emissions after combustion, DEF lets the engine run at higher efficiency and power without increasing pollution.

DEF warning light stays on
Even after filling DEF, the warning doesn’t go away. This often points to a bad tank sensor or heater.

“Low DEF” or “Incorrect DEF quality” message
The tank may not be reading the fluid level or quality correctly.

Engine power reduction (derate mode)
The vehicle limits speed or power to force repair of the DEF system.

Vehicle won’t start or restart
Many diesel vehicles will lock out starting if the DEF system fails completely.

DEF level gauge not changing
The gauge stays full or empty all the time — usually a faulty level sensor inside the tank.

Check engine light related to emissions
Trouble codes linked to SCR, DEF heater, pump, or sensors.

Frozen DEF not thawing
If DEF stays frozen too long in cold weather, the tank heater may be bad.

DEF leaking or crystallized residue
White crust around the tank or lines indicates leaks or internal tank damage.

Strong ammonia smell near the tank
Can indicate a leak or cracked DEF tank.