U01AC: Lost Communication With Hybrid/EV Battery Interface Control Module M

ECU lost CAN communication with hybrid/EV HV battery interface module M (BMS slave or cell-monitoring submodule).

High severity. Can I keep driving?If the car still runs, drive as little as possible and go to a specialized repair shop. Do not touch the orange cables: they carry high voltage.

Symptoms

Multiple warning lights may illuminate
Affected modules may go into limp mode or shut down
Specific functions inoperative depending on module
Diagnostic tools may show communication faults

Most likely causes

Module unpowered
Gateway module routing CAN messages incorrectly
CAN terminating resistor (120 Ω) drifted out of spec

What to check, in order

Start with the simplest. You can tick off each step.

Read the codes with a hybrid-compatible scanner and write down the freeze frame data
Make sure the battery cooling air intake is clean
Check the 12 V auxiliary battery, which can also trigger warnings
Leave any high-voltage measurements to a specialized repair shop

Good to know

Repair difficulty: hard
It can trigger limp mode.

What each character of U01AC means

U: Communication network
0: Generic code: same on every make
1: Subsystem depends on the code range
AC: Specific fault in that system

Frequently asked questions

Can I keep driving with code U01AC?

If the car still runs, drive as little as possible and go to a specialized repair shop. Do not touch the orange cables: they carry high voltage.

What causes code U01AC?

The most common causes are: module unpowered; gateway module routing CAN messages incorrectly; cAN terminating resistor (120 Ω) drifted out of spec.

Related codes

Sources

Values and procedures vary by manufacturer. If it affects brakes, steering, fuel or high voltage, see a professional. Updated: 2026-11-04. How we build these pages.