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Semi-Truck Headlight & Marker Light Control Module Failure – Flickering Lights or No Power | UpFix

When the Lights Go Out, Revenue Goes Dark

Every trucker knows how critical lights are—for visibility, safety, and DOT compliance. When a truck loses headlights, marker lights, or turn signals, it can’t run legally. That means hours or days of downtime and lost loads.

The real culprit is often not the bulbs or wiring—it’s the Lighting Control Module (LCM) or Body Control Module (BCM) that distributes power to those circuits. UpFix repairs these modules fast, restoring full lighting function so your truck and your business stay on the road.

Common Symptoms

  • Headlights or marker lights won’t turn on or off

  • Running lights flicker or dim randomly

  • Turn signals blink fast or stay solid

  • Fuse keeps blowing with no visible short

  • “Body Control Fault” or “Lighting Fault” on dash

  • Power remains on after key off

These symptoms show up frequently on Freightliner Cascadia, Kenworth T680, Peterbilt 579, Volvo VNL, and International LT models.

Typical Fault Codes

Code Description Meaning
SPN 804 FMI 5 Low-side driver open circuit Failed transistor inside module
SPN 1231 FMI 2 Voltage out of range on lighting feed Power regulator fault
SPN 639 FMI 9 Lost communication with BCM/LCM Logic or CAN driver failure

If you’ve checked fuses and bulbs but faults remain, the lighting control electronics themselves are to blame.

Why Lighting Modules Fail

  1. Overheating drivers from constant load on marker and headlight circuits.

  2. Moisture intrusion from rain or wash-downs corroding contacts.

  3. Voltage spikes damaging MOSFETs and regulators.

  4. Vibration fatigue cracking solder joints and pins.

  5. Aftermarket LED retrofits drawing improper current loads.

Once a driver or regulator fails, the LCM can lose an entire lighting bank or trigger false faults across the system.

Replacement vs. UpFix Repair

Dealers typically quote $1,200–$2,000 for a new lighting or body control module—plus diagnostics and programming. Back-orders can add weeks to the repair.

UpFix repairs your original module for a fraction of the cost. Your VIN and lighting configuration stay intact, and the unit comes back plug-and-play ready.

How UpFix Repairs Lighting Modules

  1. Electronic testing of each lighting output channel and relay driver.

  2. Replacement of failed transistors, regulators, and MOSFETs.

  3. Repair of cracked solder joints and burnt traces under microscope.

  4. Cleaning and conformal coating for moisture resistance.

  5. Bench load testing of headlight, marker, and signal outputs under real-world current.

Each unit is heat-cycled and vibration-tested to ensure dependable performance on the road.

Real-World Example

A 2020 Kenworth T680 arrived with flickering headlights and a recurring SPN 804 FMI 5. Bulbs and wiring tested fine—the problem was a burnt low-side driver chip inside the BCM. UpFix replaced the component, reinforced the circuit, and returned the unit within 48 hours. After installation, all lights operated normally and the fault cleared instantly.

Supported Modules

  • Freightliner / Western Star – SAM Cab & Chassis Lighting Modules

  • Kenworth / Peterbilt – Lighting Control and Smart Junction Boxes

  • Volvo / Mack – VECU Lighting and Body Controllers

  • International / Navistar – LCM and BCM Assemblies

Why Fleets and Operators Choose UpFix

  • Up to 80 percent cheaper than new replacement

  • 24–48-hour turnaround minimizes downtime

  • No programming required—VIN and configuration stay intact

  • DOT-compliant repairs restore lighting safety standards

  • Eco-friendly repair reduces electronic waste

When lighting fails, your truck can’t legally move. UpFix keeps you lit, legal, and earning.

Repairing for Safety and Sustainability

Every repaired lighting module saves about 2 lbs of electronic waste and prevents new manufacturing emissions. Repairing is a smarter, greener way to keep your fleet safe and compliant without breaking the budget.

When your truck is down, your business is losing money.
Get back on the road fast with UpFix.
Send your part to UpFix for professional testing and repair.
We’ll restore your lighting control module to like-new condition, save you up to 80 percent compared to buying new, and have you earning again in as little as 24–48 hours.