01 Fix That Check Engine Light Today: 5 Simple Sensor Cleaning… • 02 Exploring Mitsubishi's Popular Engines: Performance, Specif… • 03 8 Factors That Contribute to High Fuel Consumption • 04 High Horsepower vs High Torque: Which One is Better for You… • 05 This Is How to Test a Throttle Position Sensor Without Spec… • 06 P0025 B camshaft position - timing over retaded (Bank 2) • 07 How Bad Is It to Drive with a Leaking Head Gasket? • 08 How to Become a Pilot: A Guide for Aspiring Aviators • 09 The Critical Engine Element That Secretly Pushes Your Fuel … • 10 The Toyota Hilux: Engine Options and Reviews for Durability… •

DTC codes / U01xx · Lost communication with a control module / U01A1

U01A1Lost Communication With Hybrid/EV Battery Interface Control Module B

U01A1 is a generic network communication code indicating that one control module on the vehicle's high-speed data bus has stopped receiving messages from the Hybrid/EV Battery Interface Control Module B. The module that set the code detected a loss of the periodic serial data messages it expects from that battery interface module, not necessarily a failure of the battery pack itself.

SystemNetwork
Code typeGeneric (all makes)
SeverityHigh
Fix difficultyProfessional
AreaLost communication with a control module

Symptoms you may notice

  • Illumination of the hybrid system warning light or service vehicle soon message
  • The vehicle may refuse to start or enter Ready mode because the high-voltage battery contactors are not commanded closed
  • Reduced power, limp-home mode, or a sudden transition to gasoline engine only operation
  • The high-voltage battery state of charge gauge may read blank, zero, or behave erratically
  • Charging may not initiate on plug-in models, or the charge port indicator may show a fault
  • Often no symptoms besides the warning light if the fault is intermittent and communication recovers

Common causes, most likely first

  1. A loose, corroded, or backed-out terminal in the battery interface module B connector
  2. Damaged or chafed wiring in the CAN (Controller Area Network) bus pair between the interface module and the rest of the network
  3. Failed Hybrid/EV Battery Interface Control Module B internal communication circuitry
  4. Low 12-volt battery voltage or a poor ground causing the module to drop offline intermittently
  5. A shorted or open CAN bus segment elsewhere on the network pulling down communication
  6. Water intrusion or heat damage affecting the module or its harness in the battery pack area
  7. A software or calibration issue in the interface module requiring reprogramming

How to diagnose and fix it

  1. Verify the 12-volt battery state of charge and perform a voltage drop test on the module's power and ground circuits before condemning any component
  2. Inspect the battery interface module B connector for corrosion, loose pins, and pushed-out terminals, then reseat it firmly
  3. Check the CAN bus resistance across the diagnostic connector and compare with specification to confirm the network is intact
  4. Perform a wiggle test on the harness while watching live communication status to expose an intermittent open or short
  5. Consult the manufacturer's service information for the specific module location and follow the pinpoint test for loss of communication
  6. If wiring and power are confirmed good, substitute a known-good module or perform the manufacturer's module replacement and programming procedure
  7. Clear the code and verify communication is restored across several drive cycles before returning the vehicle

What the battery interface module code hides

  • This code is set by the module that lost the message, not by the battery interface module itself, so the fault may live anywhere on the bus rather than at the battery.
  • A weak 12-volt battery is a frequent culprit because the interface module may boot but drop off the network under cranking voltage sag, setting U01A1 without any hard failure.
  • Technicians often replace the battery interface module only to find a chafed CAN wire in the harness near the pack, so always wiggle-test the harness before condemning the module.
  • Other modules on the same bus can set companion communication codes, and a single shorted module elsewhere can pull the entire network down and mimic a failed interface module.
  • After module replacement, many vehicles require calibration or initialization of the battery interface, and skipping that step will leave the code returning immediately.

Can I keep driving with U01A1?

Do not continue driving normally because a hybrid or electric vehicle with a lost battery interface module may lose high-voltage contactor control, shut down propulsion, or fail to restart. Move the vehicle to a safe location and have it towed or diagnosed before attempting further trips.

Tools that help

Tools we recommend are in the Auto Veteran shop on Amazon. As an Amazon Associate we earn from qualifying purchases.

Guidance is general and written for all OBD-II vehicles. Exact tests and values vary by model, so confirm the fault before replacing parts.

Running on fumes Free guides aren't free to make. Top up the tank. Any amount, one time. Fuel the Garage