--- title: DEF / SCR Control Module Faults in Semi-Trucks | UpFix description: DEF or SCR fault codes? UpFix repairs semi-truck DEF control modules to clear derate mode and restore emissions performance affordably. --- [Skip to content](https://info.upfix.com/kb/def-/-scr-control-module-faults-in-semi-trucks-upfix#main-content) English Show submenu for translations ![logo (2)](https://info.upfix.com/hs-fs/hubfs/logo%20(2).png?width=189&height=56&name=logo%20(2).png) Open main navigation Close main navigation - English Show submenu for translations - Website Website Hello. How can we help you? - There are no suggestions because the search field is empty. 1. [UpFix FAQ's](https://info.upfix.com/kb?hsLang=en) 2. [Commercial Truck](https://info.upfix.com/kb/commercial-truck?hsLang=en) # 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](https://www.upfix.com/product-category/other/semi-truck-electronics-repair/)** 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. - [FAQs](https://info.upfix.com/kb/faqs?hsLang=en#main-content) - [Shipping](https://info.upfix.com/kb/faqs?hsLang=en#shipping) - [How It Works](https://info.upfix.com/kb/faqs?hsLang=en#how-it-works) - [Warranty and Returns](https://info.upfix.com/kb/faqs?hsLang=en#warranty-and-returns) - [General](https://info.upfix.com/kb/faqs?hsLang=en#general) - [Automotive](https://info.upfix.com/kb/automotive?hsLang=en#main-content) - [ABS Module](https://info.upfix.com/kb/automotive?hsLang=en#abs-module) - [LCD Display](https://info.upfix.com/kb/automotive?hsLang=en#lcd-display) - [Instrument Cluster](https://info.upfix.com/kb/automotive?hsLang=en#instrument-cluster) - [Climate Control](https://info.upfix.com/kb/automotive?hsLang=en#climate-control) - [Mileage Odometer Programming](https://info.upfix.com/kb/automotive?hsLang=en#mileage-odometer-programming) - [ECU/ECM Engine Control](https://info.upfix.com/kb/automotive?hsLang=en#ecu-ecm-engine-control) - [TCM Transmission Control](https://info.upfix.com/kb/automotive?hsLang=en#tcm-transmission-control) - [PCM Power-train Control](https://info.upfix.com/kb/automotive?hsLang=en#pcm-power-train-control) - [Semi-Truck Electronics Repair](https://info.upfix.com/kb/automotive?hsLang=en#semi-truck-electronics-repair) - [FICM Fuel Injector Control Module Repair](https://info.upfix.com/kb/automotive?hsLang=en#ficm-fuel-injector-control-module-repair) - [Power Steering Module](https://info.upfix.com/kb/automotive?hsLang=en#power-steering-module) - [TIPM & Fuse Box](https://info.upfix.com/kb/automotive?hsLang=en#tipm-fuse-box) - [BCM Body Control Module](https://info.upfix.com/kb/automotive?hsLang=en#bcm-body-control-module) - [LCM Light Control Module](https://info.upfix.com/kb/automotive?hsLang=en#lcm-light-control-module) - [ECU Programming](https://info.upfix.com/kb/automotive?hsLang=en#ecu-programming) - [Tail Light Repair](https://info.upfix.com/kb/automotive?hsLang=en#tail-light-repair) - [Tesla Wall Charger](https://info.upfix.com/kb/automotive?hsLang=en#tesla-wall-charger) - [Warranty For Automotive Parts](https://info.upfix.com/kb/automotive?hsLang=en#warranty-for-automotive-parts) - [Tech Support - Auto](https://info.upfix.com/kb/automotive?hsLang=en#tech-support-auto) - [Battery](https://info.upfix.com/kb/automotive?hsLang=en#battery) - [Appliance](https://info.upfix.com/kb/appliance?hsLang=en#main-content) - [Dishwasher Control Board](https://info.upfix.com/kb/appliance?hsLang=en#dishwasher-control-board) - [HVAC/Furnace Control Board](https://info.upfix.com/kb/appliance?hsLang=en#hvac-furnace-control-board) - [Stove/Oven Control Board](https://info.upfix.com/kb/appliance?hsLang=en#stove-oven-control-board) - [Refrigerator Control Board](https://info.upfix.com/kb/appliance?hsLang=en#refrigerator-control-board) - [Microwave Control Board](https://info.upfix.com/kb/appliance?hsLang=en#microwave-control-board) - [Massage Chair Control Board](https://info.upfix.com/kb/appliance?hsLang=en#massage-chair-control-board) - [Warranty For Appliance Control Boards](https://info.upfix.com/kb/appliance?hsLang=en#warranty-for-appliance-control-boards) - [Freezer](https://info.upfix.com/kb/appliance?hsLang=en#freezer) - [Fitness](https://info.upfix.com/kb/fitness?hsLang=en#main-content) - [Elliptical Control Board](https://info.upfix.com/kb/fitness?hsLang=en#elliptical-control-board) - [Treadmill Control Board](https://info.upfix.com/kb/fitness?hsLang=en#treadmill-control-board) - [Stair Climber Control Board](https://info.upfix.com/kb/fitness?hsLang=en#stair-climber-control-board) - [Warranty For Fitness Control Boards](https://info.upfix.com/kb/fitness?hsLang=en#warranty-for-fitness-control-boards) - [Industrial](https://info.upfix.com/kb/industrial?hsLang=en#main-content) - [VFD Inverter/Variable Frequency Drive](https://info.upfix.com/kb/industrial?hsLang=en#vfd-inverter-variable-frequency-drive) - [Servo Driver Controller](https://info.upfix.com/kb/industrial?hsLang=en#servo-driver-controller) - [HMI](https://info.upfix.com/kb/industrial?hsLang=en#hmi) - [Warranty For Industrial Equipment](https://info.upfix.com/kb/industrial?hsLang=en#warranty-for-industrial-equipment) - [Agriculture](https://info.upfix.com/kb/agriculture?hsLang=en#main-content) - [General](https://info.upfix.com/kb/agriculture?hsLang=en#general) - [Commercial Truck](https://info.upfix.com/kb/commercial-truck?hsLang=en) - [Commercial Laundry](https://info.upfix.com/kb/commercial-laundry?hsLang=en) [![Chill listening crop-3](https://info.upfix.com/hs-fs/hubfs/VECTOR%20-%20UpFix%20Registered%20logo-%20transparent.png?width=168&height=55&name=VECTOR%20-%20UpFix%20Registered%20logo-%20transparent.png "Chill listening crop-3")](http://upfix.com) UpFix FAQ's Copyright © 2026, MyAirbags