01 Why Engine Knock Happens and How It Affects Performance • 02 Top 5 Family-Friendly Buses for Comfortable Travel • 03 B0040 - Left Front or Driver Side Deployment Loop Resistanc… • 04 This Is How to Fix Sensor-Related Fuel Consumption Problems… • 05 Mechanics Shocked At How Many Cars Are Destroyed By Owners … • 06 Drivers Are Accidentally Destroying Their Cars By Trusting … • 07 Why disc and drum brakes are used in the front and rear res… • 08 What’s Dripping Under My Car? Identify Fluid Leaks By Colour • 09 Understanding the Mass Airflow Sensor: How It Works and Its… • 10 Experts Warn That A Small Everyday Driving Mistake Could En… •

DTC codes / U01xx · Lost communication with a control module / U01AC

U01ACLost Communication With Hybrid/EV Battery Interface Control Module M

This code means the main engine or hybrid control computer has lost the digital communication link with the battery interface control module that manages one section of the high-voltage hybrid or electric vehicle battery pack. The computer detected that expected messages from that module stopped arriving over the vehicle's communication network for a set period, so it flags the module as missing rather than reporting a battery cell or voltage fault.

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

Symptoms you may notice

  • The check engine light or hybrid system warning light illuminates steadily on the dashboard.
  • The vehicle may enter a reduced-power limp mode or refuse to start or shift into gear.
  • The high-voltage battery state of charge gauge may read blank, zero, or behave erratically.
  • The hybrid system may shut down or fail to engage, leaving only the gasoline engine running on a hybrid vehicle.
  • Often no symptoms besides the warning light, with the vehicle driving normally because the fault is intermittent.
  • Other unrelated warning lights may appear if the same communication network carries several modules.

Common causes, most likely first

  1. A loose, corroded, or damaged electrical connector at the battery interface control module or its harness.
  2. A break or chafe in the communication network wiring between the module and the rest of the vehicle.
  3. A failed or internally faulted battery interface control module.
  4. Low 12-volt battery voltage or a poor ground connection causing the module to drop offline.
  5. A communication network fault such as a shorted data line or a failed terminating resistor elsewhere on the bus.
  6. Water intrusion or corrosion into the module housing or connector, common in battery compartments.
  7. A software or calibration issue in the module or the control computer that disrupts messaging.

How to diagnose and fix it

  1. Verify the code with a scan tool and record all stored and pending codes before clearing anything.
  2. Check the 12-volt battery condition and charging system, then inspect and clean the module's ground connections.
  3. Locate the battery interface control module and inspect its connectors and harness for corrosion, looseness, or damage.
  4. Perform a voltage drop and continuity test on the module's power, ground, and communication network wiring.
  5. Check the communication network for shorts or opens with an oscilloscope or network tester, including terminating resistors.
  6. If wiring and power are good, substitute a known-good module or perform the manufacturer's module communication self-test.
  7. After repair, clear codes and verify communication returns and stays stable through a road test and key cycles.

Communication faults are not always module failures

  • A weak or failing 12-volt battery is one of the most common causes of this code because the module cannot boot or stay online reliably.
  • Corroded pins inside a connector that looks fine from the outside often cause intermittent loss of communication, especially after temperature swings.
  • This code does not mean the high-voltage battery itself is bad; it only reports that the module managing it stopped talking.
  • A shorted data line from an unrelated module on the same network can take down communication with the battery interface module and set this code.
  • Replacing the module without checking power, ground, and network integrity frequently leads to a repeat failure and unnecessary expense.

Can I keep driving with U01AC?

If the vehicle drives normally and only the warning light is on, you can usually drive it to a repair shop, but avoid long trips and heavy loads. If the vehicle is in limp mode, will not start, or the hybrid system has shut down, stop driving and have it towed because continued operation may strand you or cause further electrical damage.

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