4086727C1 | Genuine International® Def Tank 7 Gal Without Sensor QLS 25.5L

The purpose of using a DEF tank in a truck is to reduce harmful exhaust emissions so the truck can:

• Meet government emissions laws

• Run cleaner and more efficiently

• Avoid heavy fines, shutdowns, or restricted operation

Without a DEF system, most modern diesel trucks cannot legally operate.

Dashboard warning signs (first clues)

These usually show up before the truck limits power:

• “DEF Level Low” (even after filling)

• “Poor DEF Quality”

• “Service DEF System”

• Check Engine Light tied to DEF/SCR codes

• Countdown messages like:

  • “200 miles until speed limited”

  • “No restart in ___ miles”

⚠️ If warnings stay on after refilling, that’s a red flag.

Engine derate / power loss

A bad DEF tank or sensor can cause:

• Reduced horsepower

• Sluggish acceleration

• Speed limited to 55, 25, or even 5 mph

• Truck refusing to restart after shutdown

This happens even if the engine itself is perfectly fine.

False DEF readings (very common)

Bad DEF tank sensors can:

• Show empty when full

• Jump around randomly

• Stick at one level

• Trigger “poor quality” with fresh DEF

This is one of the most common DEF tank failures.

Cold-weather problems

If the DEF tank heater fails:

• DEF stays frozen too long

• Warning lights stay on after startup

• Truck derates in cold temps

• Heater fault codes appear

DEF is supposed to thaw within minutes—if not, something’s wrong.

DEF leaks or crystallization

Look for:

• White, chalky residue around tank, lines, or injector

• Strong ammonia smell

• Wet spots under the truck (clear liquid)

Crystallized DEF can:

• Block lines

• Kill sensors

• Damage the pump

Trouble codes related to DEF tank

Common fault areas:

• DEF level sensor

• Quality sensor

• Temperature sensor

• Tank heater

• DEF pump inside tank

Even one failed sensor can trigger a full derate.

Poor DEF quality warning (when DEF is fine)

This usually means:

• Bad quality sensor

• Contaminated tank (fuel, water, dirt)

• DEF concentration reading wrong

Fresh DEF + warning still on = tank/sensor issue, not the fluid.