U01A1: Lost Communication With Hybrid/EV Battery Interface Control Module B

What it means

Communication with Hybrid/EV Battery Interface Control Module B was not received as expected. This code does not identify the cause or specify the communication network used.

At a glance

• Usually lights the warning light of the system that lost communication
• The engine or gearbox may go into limp mode

How it works

Hybrid/EV Battery Interface Control Module B is a distinct battery-system controller that exchanges information with other vehicle modules. U01A1 sets when expected messages from interface module B are missing; it does not establish that modules A, C, or D have failed, or specify which network carries the messages.

Common causes

Most likely first.

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

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

How to diagnose it

1. Save freeze-frame data and scan all modules, checking whether communication loss is specific to interface module B or shared across the battery system.
2. Note any battery or high-voltage warnings, reduced-power behavior, disabled features, or failure to enter ready mode.
3. Visually inspect accessible low-voltage connectors and wiring for interface module B for looseness, corrosion, water, or damage.
4. Check the module's low-voltage fuse, power, and ground circuits using the manufacturer's procedure.
5. Have the gateway routing and network wiring checked, and leave high-voltage interlock testing to a technician following the manufacturer's safety procedure.

Common fixes

• Repair damaged or corroded accessible wiring and connector terminals for interface module B.
• Restore module B's low-voltage power or ground and resolve the cause if a fuse has failed.
• Correct a gateway routing or configuration fault affecting module B.
• Repair a confirmed network or interlock fault using the vehicle's specified procedure.

Can I keep driving?

If the vehicle shows a high-voltage warning, reduced propulsion, or abnormal battery behavior, stop in a safe place and do not continue driving. Otherwise, arrange prompt diagnosis because battery functions may be limited; do not access high-voltage wiring or components.