Why is my truck derated to 5 mph?

Aftertreatment faults — often SCR, DEF or sensor problems — trigger inducement derates. Diagnosis finds which component is at fault.

Can bad DEF cause emissions codes?

Yes. Diluted, contaminated or old DEF causes SCR efficiency codes and derates. Testing DEF quality is one of the first checks when SCR faults show up.

I replaced the NOx sensor and the code came back. Why?

The sensor may not have been the cause. Exhaust leaks, dosing problems or SCR issues can produce the same codes.

What causes DEF crystals?

DEF crystals form where DEF leaks and dries — at line connections, the doser or the tank. Crystal buildup can restrict dosing and trigger SCR faults, so the leak source gets repaired, not just cleaned.

Glendive, Montana

Aftertreatment & Emissions Diagnostics

Aftertreatment faults are one of the most common reasons a modern diesel truck goes into derate. The system includes the DOC, DPF, EGR, and SCR with DEF dosing, all monitored by NOx, temperature and pressure sensors. One bad reading can shut the truck down to limp mode even if the components are fine. Mondak diagnoses aftertreatment faults by testing: checking sensor readings against actual conditions, performing DEF quality and dosing checks, inspecting DEF lines and heaters, reviewing regen history and running system tests with manufacturer software. We fix the cause instead of clearing codes.

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Repair

WHAT WE SERVICE

Work tied to the actual fault.

Upstream and downstream NOx sensor diagnosis
Exhaust temperature and pressure sensor testing
DEF quality (concentration) testing
DEF dosing, pump, lines and heater checks
SCR efficiency and conversion faults
Regeneration and DPF diagnosis

COMMON NEEDS

Derate or 5 mph limit from an emissions fault
Low SCR efficiency or NOx conversion codes
DEF warning lamp or poor DEF quality
DEF crystals around the doser or lines
NOx sensor codes returning after replacement
Regen won't start or complete

THE MONDAK APPROACH

Diagnose first. Repair with purpose.

We start with the data: compare NOx readings, check temperatures, and run a dosing test. DEF concentration gets tested with a refractometer, because bad DEF can cause low efficiency codes. Sensors are replaced only after readings are confirmed wrong, and root causes like exhaust leaks or engine problems are checked first.

Code reads and some tests can be done in the field. Deeper diagnosis and repairs are done at the Glendive shop.

RELEVANT EQUIPMENT & BRANDS

CumminsDetroitPACCARInternationalVolvoMack

Brand references indicate service familiarity, not authorized dealer or factory status.

COMMON QUESTIONS

Questions about aftertreatment & emissions diagnostics

DPF Cleaning & Repair EGR Cleaning & Repair Truck Diagnostics Semi Truck Repair Advanced Diagnostics

GLENDIVE, MONTANA

Tell us what’s down.

Share the unit, location and symptoms. We’ll help determine the practical next step.

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