If your Freightliner semi-truck is showing the SCR closed loop at maximum code, it means the emissions system has reached its limit trying to reduce NOx emissions. This is a common fault that requires immediate attention to avoid an engine derate (loss of power). If DEF light is flashing, a forced regeneration will not fix the issue. We would need to do SCR Efficiency Test.

The Freightliner Cascadia SCR system uses Diesel Exhaust Fluid (DEF) and a catalyst to reduce harmful nitrogen oxides (NOx) from the exhaust. Exhaust gases are filtered, DEF is injected, and chemical reactions inside the SCR convert pollutants into clean nitrogen and water vapor. This process keeps the truck compliant with emissions standards and prevents costly engine derates when properly maintained.

What Causes the SCR Closed Loop at Maximum Code?

This fault can be triggered by several issues in the Freightliner aftertreatment system, including:

• Bad or contaminated DEF (Diesel Exhaust Fluid)

• Faulty NOx sensors sending incorrect data

• DEF injector or pump problems

• Clogged or crystallized DEF lines inside the SCR system

• many items can fail, from a Nox sensor to an ambient temperature sensor. It all helps control SCR system.

Repair -Fitst we connect Detroit Diesel Diagnostic Link (DDDL.8).

Once connected we can start diagnosing the problem. Depending on the cause, repairs may include removing and cleaning DEF dozer and fuel dozer, replacing DEF pump filter if necessary. May need to replace Nox sensors, cleaning the DEF system, or repairing the injector/pump.

• System Inspection - DEF fluid, sensors, and SCR components are tested for proper function.

• Repairs & Replacements – This may involve replacing NOx sensors, cleaning clogged lines, replacing the DEF injector or pump, or flushing contaminated DEF. Most importantly, performing an SCR Efficiency Test is essential to raise the LARMF to 1.0 or higher. It may take 2–3 SCR efficiency tests to temporarily clear the codes.

• Code Reset - After repair, if fault code is cleared

• System Reset – Once repaired, the code is cleared and the truck is road-tested to ensure proper operation.

Time & Cost

Most repairs take 2–6 hours, depending on the severity of the issue and part availability. In some cases, if multiple sensors or components fail, it may take longer.

What You Should Do

• If your truck shows this code, it’s important to address it promptly. Continuing to drive with an SCR fault can trigger a derate, limiting engine power and leaving you stranded. Bring your truck to a qualified repair shop as soon as possible for diagnosis and repair. Engine Derate to 55 or 5 MILES PER HOUR.

LARMF is a number the truck’s computer uses to measure how well the SCR system is working.

• NH₃ = Ammonia (the active chemical in DEF that reduces emissions).

• LARMF = Load-Adaptive Reductant Multiplication Factor (basically a “correction factor”).

When you run an SCR Parked Efficiency Test, the computer checks how much ammonia is being used compared to how much should be used.

✅ If the factor is 1.0 or higher, the system is working correctly and codes will clear.

🚫 If it’s below that (like 0.6 or 0.7), the system isn’t reducing emissions enough, and the truck will stay derated until repairs are complete.

If LARMF is too low, the SCR Efficiency Test can take 3–6+ hours and won’t pass until repairs are made.

A FORCED REGENERATION WILL NOT FIX THE ISSUE !