DEF / SCR Control Module Faults in Semi-Trucks | UpFix

Why Does My Semi-Truck Say “Service DEF System”?

If your truck displays DEF warnings or goes into derate mode, the DEF or SCR control module may be at fault. This module regulates DEF injection and monitors exhaust sensors. When it fails, your truck may restrict speed or power to meet emissions compliance.

Common Signs of Failure

• DEF light on constantly

• Truck derated to 5 mph

• DEF tank heater or pump codes

• SCR system fault codes (P20EE, P207F, etc.)

Crystallized DEF fluid, corrosion, and voltage surges often damage control boards. Once shorted, they send false readings to the ECM.

Why Repair Over Replacement

OEM DEF modules can cost thousands and often require coding to the ECM. UpFix repairs your original module, preserving factory programming.

UpFix Process

• Visual and electrical inspection

• Replacement of damaged transistors and connectors

• Load testing on DEF system simulator

• Final verification of CAN communication

Supported Brands

Freightliner, Volvo, Kenworth, Peterbilt, and International and others.

Get your truck back on the road and keep your costs low.
Send your part to UpFix for professional testing and repair.
We’ll restore it to like-new condition, save you up to 80% compared to buying new, and make it plug-and-play ready when returned.