U029A: Lost Communication With Hybrid/EV Battery Pack Sensor Module

What it means

The vehicle bus reports loss of communication with the high-voltage battery pack sensor module. Cell-voltage and temperature data needed for state-of-charge and state-of-health calculation are missing.

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

The hybrid or EV battery pack sensor module reports cell-voltage and temperature information used to estimate battery condition and manage the pack. U029A sets when the vehicle network stops receiving messages from that module, leaving other controllers without those measurements.

Common causes

Most likely first.

1. Bus wiring open or shorted
2. Module without power or ground
3. Module failed

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. Scan all modules and record freeze-frame data and related high-voltage battery or network codes to establish whether communication loss is isolated to the pack sensor module.
2. Check 12-V battery condition and module availability on the scan tool, since low 12-V supply can prevent the sensor module from waking.
3. Visually inspect accessible low-voltage connectors and bus wiring serving the battery pack sensor module for damage, corrosion, or poor seating.
4. Using vehicle wiring information, check the module's power and ground circuits under load against the manufacturer's specification.
5. Test the communication circuit for opens, shorts, or connection faults, and evaluate the module only after its wiring and supplies pass.

Common fixes

• Repair a damaged or poorly seated connector in the pack sensor module's communication wiring.
• Repair an open or short in the bus wiring to the battery pack sensor module.
• Restore the module's failed power or ground connection.
• Replace the battery pack sensor module if its supplies and network circuit are correct but it remains offline.

Can I keep driving?

Do not continue normal driving if a hybrid or EV battery warning, reduced-power mode, or shutdown occurs; missing pack measurements can cause the vehicle to limit or stop propulsion. Have the fault diagnosed promptly, and do not access high-voltage components yourself.