Damaged, corroded, or loose wiring or connector pins at the battery interface module; Open or shorted controller area network (CAN) bus wiring between the interface module and the network; Failed Hybrid/EV Battery Interface Control Module K or its internal power supply; Blown fuse or poor ground connection supplying the interface module; Low 12-volt battery voltage or charging system fault causing modules to drop offline; Water intrusion or heat damage affecting the module or its harness; Aftermarket electronics improperly tapped into the communication bus

DTC codes / U01xx · Lost communication with a control module / U01AA

U01AALost Communication With Hybrid/EV Battery Interface Control Module K

U01AA is a generic communication fault indicating that one or more control modules have lost contact with the Hybrid/EV Battery Interface Control Module K over the vehicle's high-speed data network. The module that set the code expected regular status messages from the battery interface module and stopped receiving them for a calibrated time, so it flagged the communication loss.

SystemNetwork

Code typeGeneric (all makes)

SeverityMedium

Fix difficultyProfessional

AreaLost communication with a control module

Symptoms you may notice

Illumination of the check engine light, service vehicle soon message, or hybrid system warning lamp
The vehicle may refuse to enter Ready mode or may shut down shortly after start
Reduced power, limited hybrid operation, or a forced limp-home mode
Inoperative or erratic state-of-charge and battery temperature displays
Often no symptoms besides the warning light if the loss is intermittent
Multiple unrelated warning lights if the module resides on a shared communication bus

Common causes, most likely first

Damaged, corroded, or loose wiring or connector pins at the battery interface module
Open or shorted controller area network (CAN) bus wiring between the interface module and the network
Failed Hybrid/EV Battery Interface Control Module K or its internal power supply
Blown fuse or poor ground connection supplying the interface module
Low 12-volt battery voltage or charging system fault causing modules to drop offline
Water intrusion or heat damage affecting the module or its harness
Aftermarket electronics improperly tapped into the communication bus

How to diagnose and fix it

Scan all modules and record every stored code, then verify battery voltage and charging system output
Inspect the battery interface module connectors for corrosion, pushed pins, and loose terminals
Check the module's power and ground circuits with a multimeter under load
Measure CAN bus resistance and voltage to rule out an open or shorted data line
Perform a wiggle test on the harness while watching live communication status on the scan tool
If wiring and power are good, substitute a known-good module or perform a module communication self-test
Clear codes and verify the fault does not return after a road test that includes a key-off sleep cycle

Why the battery interface module drops offline

A weak or failing 12-volt battery is a frequent culprit because the interface module may brown out during cranking or sleep cycles and set U01AA without any hard wiring fault.
The code can be stored by a module other than the battery interface module itself, so always check which module reported the loss before condemning the battery pack electronics.
Intermittent U01AA often appears only after the vehicle sits overnight, pointing to a parasitic draw or a module that fails to wake on the bus.
A single unplugged connector at the interface module can set this code along with several other U-codes, so review the full code list before replacing parts.
Aftermarket alarms or dash cameras tapped into CAN wiring commonly disturb bus voltage and trigger communication codes like this one.

Can I keep driving with U01AA?

If the vehicle still starts and drives normally, limit driving to a short trip to a repair facility and avoid high-voltage charging or long journeys. If the vehicle will not enter Ready mode or repeatedly shuts down, do not drive it and have it towed.

Tools that help

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