U0299: Lost Communication With DC/DC Converter Control Module B

What it means

The vehicle bus reports loss of communication with DC/DC converter module B. The converter that supplies the 12-V system from the high-voltage battery cannot be commanded; 12-V load shedding may begin.

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 network expects messages from DC/DC converter control module B, which manages conversion from the high-voltage battery to the 12-V electrical system. U0299 sets when those messages are missing, so the vehicle may be unable to command converter B and may reduce 12-V electrical loads.

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 save freeze-frame and communication-fault data to see whether U0299 began with other network faults or a low 12-V supply.
2. Check the 12-V battery condition and charging-system data, since unstable 12-V power can interrupt module communication.
3. Inspect module B's accessible connectors and bus wiring for looseness, corrosion, damage, or signs of overheating.
4. Use the wiring information for the vehicle to check module B's power and ground circuits under load against the manufacturer's specification.
5. Test the bus circuit to module B for opens, shorts, or poor connections, then assess the module only after its power, ground, and network wiring pass.

Common fixes

• Repair a loose, corroded, or damaged connector at converter module B.
• Repair an open or short in the bus wiring serving module B.
• Restore a faulty power or ground connection to module B.
• Replace converter control module B if its supplies and network circuits are sound and it still does not communicate.

Can I keep driving?

Avoid relying on the vehicle for routine travel if 12-V warnings, electrical functions dropping out, or reduced-power operation appear; the vehicle may shut down affected systems as 12-V power falls. Arrange prompt diagnosis, and stop somewhere safe if steering, braking assistance, or other essential functions are affected.