P0B29: Hybrid Battery "B" Voltage Low
7 vehicles · 2 makes · $200-$6,500 repair
Most Reported On
P0B29 is an OBD-II diagnostic trouble code meaning “Hybrid Battery "B" Voltage Low.” The battery energy control module monitors every cell block in the high-voltage or 48V battery pack to confirm each is contributing equally. P0B29 sets when the voltage measured across the block designated "B" falls below the calibrated threshold, or falls too far below the other blocks. In practice it means one section of the pack is weak, degraded, or not being sensed correctly - it is a cell-group-level complaint, not a whole-pack complaint. This code is most commonly reported on 2021-2025 Mercedes-Benz S-Class (S500, S580), 2020-2025 Mercedes-Benz GLS (GLS450, GLS580), and 2019-2025 Mercedes-Benz AMG GT 4-Door (AMG GT 43, AMG GT 53, AMG GT 63 S E Performance), plus 4 other vehicles, with repair costs ranging from $200 to $6,500.
Common Causes
Typical Repair Cost
Based on 7 documented vehicle-specific issues. Actual cost depends on root cause and vehicle.
Diagnostic Tools
This is a powertrain code, so a basic OBD-II reader can read it. These read P0B29.
ANCEL AD310 Classic OBD-II Scanner
by ANCEL
A simple code reader for engine (P) codes on many compatible OBD-II vehicles. It does not support every vehicle or hybrid/EV application, and it does NOT read body, chassis or network codes; confirm exact-vehicle compatibility before buying.
- Read & clear engine (P) codes
- View freeze frame data
- I/M readiness status
- No batteries or app needed
BlueDriver Pro Bluetooth Scanner
by BlueDriver
Bluetooth scanner with a companion app that reads beyond the engine module — ABS, SRS and transmission — plus manufacturer-specific codes on many vehicles.
- Enhanced diagnostics (ABS, SRS, transmission)
- Repair Reports with verified fixes
- Smog check readiness
- Free app (iOS & Android)
LAUNCH CRP123X V3.0 Elite OBD-II Scanner
by LAUNCH
Handheld scanner covering the four major systems (engine, transmission, ABS, SRS) with live data streaming and graphing.
- Engine, transmission, ABS, SRS diagnostics
- Live data stream & graphing
- AutoVIN for vehicle identification
- Free lifetime updates via Wi-Fi
Autel MaxiCOM MK808S Diagnostic Tool
by Autel
Shop-level tablet with all-system coverage and bi-directional control, so you can command a module rather than only read it.
- All-system diagnostics (25+ modules)
- Bi-directional control & active tests
- Oil reset, EPB, BMS, TPMS, injector coding
- 7-inch touchscreen with Android OS
What you can do yourself, and what needs a shop
Reading the code, clearing it, and watching whether it returns is a driveway job with the right scanner. These steps are not:
- Module programming or software updates — needs the manufacturer software or a J2534 pass-through with a paid subscription.
- Replacing a control module — most are VIN-programmed and immobilizer-registered, so a new one does nothing until it is coded to your car.
- Airbag/SRS component work — a stored fault can deploy a live device.
If the vehicle is still under warranty, check for an open recall or technical service bulletin before buying anything — the repair may already be covered.
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Vehicles Affected (7)
48V Mild Hybrid Battery Issues
Battery Drain from 48V System
48V Mild-Hybrid Battery Degradation and Faults
48-Volt Integrated Starter-Generator / DC-DC Converter Failure Causing Warning Messages, Limp Mode, or No Restart
48V Mild-Hybrid / Integrated Starter-Generator Failure Causing No-Start and Power Loss
48V Integrated Starter-Generator / DC-DC Converter Failure Causing Sudden Power Loss and No-Restart
FAQ
What does P0B29 mean?
P0B29 stands for "Hybrid Battery "B" Voltage Low." The battery energy control module monitors every cell block in the high-voltage or 48V battery pack to confirm each is contributing equally. P0B29 sets when the voltage measured across the block designated "B" falls below the calibrated threshold, or falls too far below the other blocks. In practice it means one section of the pack is weak, degraded, or not being sensed correctly - it is a cell-group-level complaint, not a whole-pack complaint.
What are the most common causes of P0B29?
The most common causes of P0B29 are: Degraded or failing cell block within the battery pack, Poor connection at a cell-group busbar or voltage-sense connector, Faulty voltage-sense wiring harness inside the pack, Failed battery energy control module / cell supervision circuit, Battery pack imbalance from long storage or repeated deep discharge, On 48V mild-hybrid systems: failing 48V lithium battery or its DC/DC converter. The specific cause varies by vehicle.
How much does it cost to fix P0B29?
Repair costs for P0B29 range from $200 to $6,500, depending on the vehicle and root cause.
Which vehicles are affected by P0B29?
Au7o has documented P0B29 across 7 vehicle models from 2 manufacturers: Audi, Mercedes-Benz.
Content compiled with AI assistance using NHTSA complaints, TSBs, and owner reports. May contain errors. Always verify with your vehicle's service manual.