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Semi-Truck ABS / EBS Control Module Failure – Brake System Fault SPN 791 / U0121 | UpFix

When the Brake Fault Light Means Lost Time

When the ABS or traction control light comes on, most drivers feel that sinking feeling—because a brake system fault means more than safety risk. It means DOT downtime, failed inspections, and missed loads.

For fleet operators, that’s money off the clock. And often, the real problem isn’t a wheel sensor or wiring—it’s the ABS / EBS electronic control module itself. UpFix repairs these modules quickly and reliably, getting your trucks back to work and revenue flowing again.

Common Symptoms

  • Constant or flashing ABS / EBS warning light

  • Brake system fault message on dash

  • Loss of traction or stability control

  • No communication with the ABS ECU (U0121)

  • Intermittent braking imbalance or false activation

  • Power or ground fault codes that come and go

These failures are common on Freightliner Cascadia, Kenworth T680, Peterbilt 579, Volvo VNL, International LT, and other heavy-duty tractors with Bendix or WABCO systems.

Typical Fault Codes

Code Description Meaning
SPN 791 FMI 5 Wheel-speed sensor open circuit Internal circuit driver fault
SPN 803 FMI 9 Power supply error Voltage regulator failure
U0121 Lost communication with ABS control module CAN transceiver or logic fault
SPN 636 FMI 2 Valve driver circuit abnormal Shorted output stage

If wiring and sensors check out but codes persist, the ABS / EBS module is the likely culprit.

Why ABS / EBS Modules Fail

  1. Heat and vibration crack solder joints on power transistors.

  2. Corrosion from road salt or moisture seeps into the housing.

  3. Voltage spikes during jump-starts or alternator surges burn regulator circuits.

  4. Aging components weaken signal accuracy and power control.

  5. Connector stress from repeated service loosens board traces.

Eventually the module loses signal stability—triggering random brake and traction faults.

Replacement vs. UpFix Repair

New ABS / EBS modules often cost $1,500–$3,000 and require dealer programming and bleeding of the system. Back-orders can stretch downtime to weeks.

UpFix repairs your original module for a fraction of the cost. You keep your factory programming and brake calibration. No dealer visit. No re-learn. Plug it back in and go.

How UpFix Repairs ABS / EBS Control Modules

  1. Full electronic diagnostics of power, ground, and CAN communication.

  2. Replacement of burnt regulators, drivers, and capacitors.

  3. Micro-soldering repair of cracked joints and corroded connectors.

  4. Sealing and conformal coating for long-term moisture protection.

  5. Bench testing on an ABS simulation rig to verify wheel-speed input and valve response.

Every unit is stress-tested under thermal and vibration conditions to ensure durability on the road.

Real-World Example

A 2018 Freightliner Cascadia displayed SPN 791 and U0121 faults. The dealer recommended replacing the WABCO ABS module—quoted at $2,400 plus programming. UpFix diagnosed a failed 5-volt regulator and CAN driver, repaired both, resealed the board, and shipped it back in 48 hours. The truck cleared all faults immediately on reinstallation and passed inspection the next morning.

Supported Brands and Systems

  • Bendix EC-80 / EC-90 ABS / EBS

  • WABCO ABS / EBS Systems

  • Detroit Brake Control Modules

  • PACCAR / Kenworth / Peterbilt ABS Controllers

  • Volvo / Mack VECU-Integrated Brake Electronics

Why Operators and Fleet Owners Choose UpFix

  • Up to 80 percent savings vs. new modules

  • Fast 24–48-hour turnaround

  • No programming required—VIN and configuration stay intact

  • Restores safety and compliance for DOT inspection

  • Environmentally responsible—each repaired ABS ECU saves ~2 lbs of electronic waste

When trucks stop moving, profits stop too. UpFix helps you stay compliant, safe, and productive.

Repairing for Safety and Sustainability

Repairing isn’t just cheaper—it’s smarter.
Each ABS / EBS repair keeps heavy metals, circuit boards, and plastic housings out of landfills while eliminating the carbon footprint of new manufacturing. Sustainable repair supports your business and the planet.

 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 ABS / EBS 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.