P0B29 on Mercedes-Benz: How to Diagnose & Fix
Hybrid Battery "B" Voltage Low
6 Mercedes-Benz models · 6 documented issues · 3 critical · $200-$6,500 repair
P0B29 on Mercedes-Benz by model: symptoms, diagnosis and fix
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.
Common Symptoms
- Multiple electrical warning messages on dashboard
- Start-stop system disabled
How to Fix
Have the dealer diagnose with XENTRY to differentiate between a software fault and genuine battery failure. Update to the latest software calibration. If the 48V lithium-ion battery has failed, replacement runs $1,500-3,000 at the dealer.
Owner tips & cautions
- TipRequest the latest XENTRY software update before approving a battery replacement. Many 48V fault codes are resolved by software calibration alone.
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.
Common Symptoms
- Dead battery after sitting for 3-5 days
- Multiple electrical warnings on startup
How to Fix
Update the 48V battery management software at the dealer. If both batteries are deeply discharged, they may need replacement. The 12V battery is $200-400, the 48V battery is $1,500-3,000.
Owner tips & cautions
- TipIf the GLS sits for more than a week, connect a battery tender to the 12V battery under the hood. The 48V system draws from the 12V to maintain its state of charge.
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.
Common Symptoms
- EQ Boost system fault warning
- Reduced electric assist during acceleration
- ISG not restarting engine smoothly
- Start-stop system disabled
- Multiple electrical warning messages
How to Fix
Diagnose the 48V battery state of health with Mercedes Star Diagnostics. If capacity has dropped below threshold, replace the 48V lithium-ion battery pack located in the engine bay. Software calibration is required after battery replacement.
Owner tips & cautions
- WarningIf the 48V battery fails out of warranty, expect $2,000-$3,000 for replacement. The battery is separate from the main 12V battery and requires dealer-level diagnostics to properly calibrate.
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.
Common Symptoms
- 48V battery malfunction warning
- stop vehicle leave engine running message
- multiple warning lights
- vehicle will not restart
- loss of start-stop function
- reduced power
- battery not charging
How to Fix
Perform a full scan of both 12V and 48V systems with Mercedes-capable diagnostics, checking charging voltage, converter operation, and fault memory in the powertrain and energy management modules. Repairs vary by root cause but commonly involve replacing the DC-DC converter, 48V battery, wiring, or the integrated starter-generator assembly, followed by software updates and battery registration/adaptation. Because the system is safety-critical and high-voltage relative to standard 12V systems, diagnosis is best handled by a Mercedes specialist or dealer.
Owner tips & cautions
- TipDo not keep driving on repeated 48V warnings; low converter output can cascade into 12V undervoltage faults and strand the car.
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.
Common Symptoms
- vehicle will not start
- 48V battery warning
- check engine light
- reduced power
- charging system malfunction message
- engine shuts off or will not restart after auto stop/start
- vehicle requires towing
How to Fix
A proper diagnosis requires scanning both the 48V and 12V systems, checking state of charge, converter operation, and fault history in the powertrain and hybrid control modules. Repairs may include software updates, replacement of the integrated starter-generator or associated power electronics, and battery/module adaptation after installation. Because the system is high-current and model-specific, independent repair options are limited compared with dealer service.
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.
Common Symptoms
- 12V battery/charging system warning
- Check engine light or multiple warning lights
- Vehicle will not start
- Sudden loss of power or limp mode
- Engine shuts off at a stop and will not restart
- Rough restart behavior
How to Fix
Scan the vehicle for 48V powertrain and charging faults, verify 12V and 48V battery state, and inspect freeze-frame data for ISG/DC-DC converter shutdown events. Dealers typically update control-unit software and replace the failed DC-DC converter, integrated starter-generator, or related 48V components as needed. Battery support and proper post-repair coding/adaptation are important to prevent repeat faults.
Owner tips & cautions
- TipIf buying used, check service history for prior 48V battery, ISG, or DC-DC converter replacement and ask whether the latest powertrain software campaigns were completed.
- TipOwners recommend scanning with a Mercedes-capable tool rather than a generic OBD reader because many 48V faults are stored in submodules not visible to basic scanners.
P0B29 code reference
General diagnostic-code context. The code alone does not confirm a cause for your vehicle; use the applicable issue card and its sources.
Frequently asked questions
What does P0B29 mean on a 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 it is documented across 6 models: S-Class, GLS, AMG GT 4-Door, CLS-Class, C-Class, E-Class.
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 on a Mercedes-Benz?
Published repair estimates on Mercedes-Benz range from $200 to $6,500 depending on the model and root cause. If an issue mentions a recall or campaign, check your VIN with the manufacturer or NHTSA and confirm eligibility and coverage with a dealer.
Looking for P0B29 on a different make?
View P0B29 across all makes →Content compiled with AI assistance using NHTSA complaints, TSBs, and owner reports. May contain errors. Always verify with your vehicle's service manual.