--- title: Semi-Truck Lighting or Body Control Module Fault – Headlights, Markers, or Turn Signals Not Working | UpFix description: Headlights or marker lights acting up? UpFix repairs semi-truck body and lighting control modules to restore reliable operation, save money, and reduce waste. --- [Skip to content](https://info.upfix.com/kb/semi-truck-lighting-or-body-control-module-fault-headlights-markers-or-turn-signals-not-working-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) # Semi-Truck Lighting or Body Control Module Fault – Headlights, Markers, or Turn Signals Not Working | UpFix ### **When Lights Stop Listening** You flip the headlight switch—and nothing. Maybe your **headlights stay on after shutdown**, or your **marker lights flicker** like there’s a short. The issue often isn’t the bulbs or wiring—it’s the **Body Control Module (BCM)** or **Lighting Control Module (LCM)** inside your truck. These modules are the nerve center for lighting, wipers, and other accessories. They regulate voltage to multiple systems, monitor current draw, and communicate with the ECM. When they start failing, you’ll see chaos in your lighting circuits—and no amount of bulb changing fixes it. --- ### **Common Symptoms** - Headlights or marker lights won’t turn on or off - Turn signals blink rapidly or stay solid - Running lights flicker while driving - Fuse keeps blowing for no clear reason - “Body Control Fault” message on dash - Lighting relays click repeatedly even with key off These problems commonly appear in **Freightliner Cascadia**, **Kenworth T680**, **Peterbilt 579**, **Volvo VNL**, and **International ProStar** trucks. --- ### **Typical Fault Codes** | Code | Description | Meaning | | --- | --- | --- | | **SPN 804 FMI 5** | Low side driver circuit open | Burnt transistor inside BCM | | **SPN 1231 FMI 2** | Marker light voltage abnormal | Regulator or power supply issue | | **SPN 639 FMI 9** | Communication error with BCM | Module offline or logic fault | While damaged wiring can cause lighting problems, recurring codes and flickering lights after wiring checks often point to **internal BCM/LCM failure**. --- ### **Why BCMs and LCMs Fail** 1. **Heat from constant electrical load** weakens transistors and regulators. 2. **Moisture intrusion** corrodes PCB traces and connectors. 3. **Vibration and thermal cycling** crack solder joints. 4. **Voltage surges** from alternators or jump-starts damage driver circuits. 5. **High current draw from aftermarket lights** overstresses output channels. Once damaged, these modules can leak voltage to the wrong circuit or drop power to headlights entirely. --- ### **Dealer Replacement vs. UpFix Repair** A new BCM or LCM can cost **$1,000–$2,500** and may require VIN programming at a dealership. Used ones often mismatch configuration or carry the same underlying faults. **UpFix repairs your original module**, keeping its programming intact—so it’s plug-and-play on return. --- ### **How UpFix Repairs Lighting and Body Control Modules** 1. **Diagnostic test** of each lighting output channel and relay driver. 2. **Replacement of failed MOSFETs, regulators, and relay circuits.** 3. **Cleaning and re-sealing** the board to prevent moisture damage. 4. **Reflow of cracked solder joints** from vibration. 5. **Bench testing under simulated lighting load** to confirm all outputs work within spec. Every unit is validated on a power simulator that mimics headlight, marker, and turn signal loads, ensuring proper current draw and CAN communication before shipment. --- ### **Real-World Example** A 2020 Kenworth T680 showed flickering headlights and repeated **SPN 804 FMI 5** even after wiring repairs. The root cause was a failed low-side driver circuit in the BCM. UpFix replaced the driver chip, repaired corroded traces, and resealed the board. On reinstall, all lighting functions returned immediately—no programming or dealer visit required. --- ### **Supported Brands and Modules** - Freightliner / Western Star – SAM Cab / SAM Chassis Modules - Kenworth / Peterbilt – Smart Junction / LCM Units - Volvo / Mack – LCM and VECU Integration - International – BCM and Headlamp Power Modules --- ### **Savings and Sustainability** Repairing instead of replacing means: - **Up to 80% cost savings** versus new modules. - **24–48-hour turnaround** for most repairs. - **Zero reprogramming downtime.** - **Lower environmental footprint:** Each repaired BCM saves up to **2 lbs of copper and circuit waste** from landfills. By choosing repair, fleets reduce both electronic waste and the energy required to manufacture new parts. --- 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. 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