U0299: Lost Communication with DC-DC Converter B
Diagnostic trouble code U0299 indicates that the vehicle's main control module (typically the PCM or hybrid control module) has lost communication with the DC-DC converter 'B' module over the CAN bus. The DC-DC converter is a critical component in hybrid and electric vehicles that steps down high-voltage traction battery power to 12V for conventional electrical systems. The 'B' designation suggests multiple converters are present. This loss of communication prevents proper monitoring and control of the 12V system charge state and can affect overall vehicle operation.
1. Commonly Affected Makes
This is a generic OBD2 code, potentially applicable to any manufacturer using this diagnostic standard.
2. Most Common Causes
- Damaged or corroded CAN bus wiring harness to the module
- Open or short circuit in CAN communication lines (CAN-H or CAN-L)
- Power supply issue (blown fuse, faulty relay, or poor ground) to the DC-DC converter 'B' module
- Faulty CAN bus terminator or incorrect termination resistance
- Electromagnetic interference affecting signal integrity
- Incomplete or corrupted software update in the module
3. Symptoms You Will Notice
- Check Engine Light or hybrid system warning light illuminated
- 12V battery not charging properly, leading to dead battery or electrical system issues
- Reduced fuel economy or hybrid system performance
- Vehicle may not start or may enter limp mode
4. Step-by-Step Diagnosis
Scan for all stored codes and freeze frame data to confirm U0299 and related codes
Visually inspect the CAN bus wiring and connectors for damage, corrosion, or loose connections
Check power supply to the DC-DC converter 'B' module: test fuses, relays, and ground circuits
Measure CAN bus resistance at the module connector (should be ~60 ohms with termination)
Perform a network communication test to see if the module responds; if not, suspect module failure
If wiring and power are good, replace the DC-DC converter 'B' module and reprogram if necessary
5. Frequently Asked Questions
Is it safe to drive?▼
Driving with code U0299 is not recommended. The 12V system may not charge properly, leading to a dead battery, loss of electrical functions, or unexpected shutdown. In hybrid vehicles, it can also affect high-voltage system operation. Have the vehicle diagnosed and repaired promptly.
How much does it cost to fix?▼
Repair costs vary widely depending on the root cause. A simple wiring repair or fuse replacement may cost $100-$300. Replacing the DC-DC converter module itself can range from $500 to $1,500 including parts and labor. Diagnosis fees are typically $100-$200.