P0B29 on Mercedes-Benz
Hybrid Battery "B" Voltage Low
P0B29 on Mercedes-Benz vehicles indicates hybrid battery "b" voltage low. Au7o has documented this code across 6 Mercedes-Benz models — most commonly on AMG GT 4-Door, C-Class, CLS-Class. 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. Typical repair costs on Mercedes-Benz range from $200 to $6,500, depending on the specific model and root cause.
Common Causes of P0B29
- •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
P0B29 on Mercedes-Benz by Model
Mercedes-Benz AMG GT 4-Door(1 issue)
- 48V Mild-Hybrid Battery Degradation and Faults2019-2025
The 48V EQ Boost mild-hybrid system battery in AMG GT 43 and GT 53 models degrades prematurely, causing reduced electric assist, ISG (Integrated Starter-Generator) faults, and inability to use electric auxiliary functions. The E Performance plug-in models can also experience 48V system faults separate from the main high-voltage battery.
Mercedes-Benz C-Class(1 issue)
- 48V Mild-Hybrid / Integrated Starter-Generator Failure Causing No-Start and Power Loss2022-2025
The W206 C-Class uses a 48V mild-hybrid system with an integrated starter-generator (ISG), and owners have reported no-start conditions, repeated warning messages, charging-system faults, and sudden reduced-power events tied to the 48V system. Mercedes has issued service information and field actions on related hybrid electrical faults across current models using this architecture, and owner complaints describe vehicles becoming undriveable or requiring towing. Failures can involve the ISG itself, DC/DC conversion, wiring, or software/calibration issues that trigger shutdowns and battery warnings.
Mercedes-Benz CLS-Class(1 issue)
- 48-Volt Integrated Starter-Generator / DC-DC Converter Failure Causing Warning Messages, Limp Mode, or No Restart2019-2024
Owners of six-cylinder CLS 450 models have reported failures in the 48V mild-hybrid system, including the integrated starter-generator, 48V battery support components, or DC-DC converter. Symptoms include 'Stop Vehicle Leave Engine Running,' multiple warning lights, loss of charging support, disabled start/stop, reduced power, and in some cases inability to restart after shutting the car off. This issue is well documented across Mercedes models using the M256/48V architecture and appears in owner complaints and service literature.
Mercedes-Benz E-Class(1 issue)
- 48V Integrated Starter-Generator / DC-DC Converter Failure Causing Sudden Power Loss and No-Restart2021-2023
Multiple owners of facelift W213 E-Class models with the M256 mild-hybrid system reported sudden warning messages, loss of charging, reduced power, stalling, or a no-start condition traced to the 48V system. Mercedes issued service communications and there are numerous owner complaints describing failure of the integrated starter-generator or associated DC-DC converter/48V battery support hardware. When the 48V system drops offline, the vehicle may illuminate multiple warnings and can become undriveable or fail to restart.
Mercedes-Benz GLS(1 issue)
- Battery Drain from 48V System2020-2025
The 48V EQ Boost mild hybrid system causes excessive battery drain when parked. A software bug in the 48V battery management module fails to properly enter sleep mode, draining both the 48V lithium-ion and 12V lead-acid batteries.
Mercedes-Benz S-Class(1 issue)
- 48V Mild Hybrid Battery Issues2021-2025
The 48V EQ Boost mild hybrid system battery degrades or fails prematurely, triggering multiple warning messages and disabling the start-stop, electric boost, and regenerative braking functions. Software glitches also cause false battery fault warnings.
Looking for P0B29 on a different make?
View P0B29 across all makes →Frequently Asked Questions
What does P0B29 mean on Mercedes-Benz?▼
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. On Mercedes-Benz specifically, this code is documented across 6 models.
What causes P0B29 on Mercedes-Benz vehicles?▼
Common causes on Mercedes-Benz: 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. Specific causes vary by model and year — see the per-model sections below.
How much does it cost to fix P0B29 on a Mercedes-Benz?▼
Repair costs on Mercedes-Benz range from $200 to $6,500, depending on the specific model and root cause.
Which Mercedes-Benz models have P0B29 documented?▼
Au7o has documented P0B29 on 6 Mercedes-Benz models: AMG GT 4-Door, C-Class, CLS-Class, E-Class, GLS, S-Class.