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P0B29CriticalHybrid / High-Voltage Battery Management

P0B29: How to Diagnose & Fix — Hybrid Battery "B" Voltage Low

7 vehicles · 2 makes · $200-$6,500 repair

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How to diagnose and fix P0B29: most-reported vehicles

Ranked by owner reports. Each card gives the symptoms, how to confirm the cause, the fix, parts and sources for that vehicle.

Mercedes-Benz S-Class · 2021–2025

Community reported
600 owners
Link to this issueError Codes:P0A0FP0A1AP0B29P0CA7

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.
Typical repair cost:$200 - $3,000

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Based on 600 owner reports

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Community Reported
Medium Confidence600 owner reportsLast reported by owners Dec 2025Reviewed Mar 2026

Mercedes-Benz GLS · 2020–2025

Community reported
450 owners
Link to this issueError Codes:P0A0FP0A1AP0B29P0CA7

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.
Typical repair cost:$200 - $3,500

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Based on 450 owner reports

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Community Reported
Medium Confidence450 owner reportsLast reported by owners Dec 2025Reviewed Mar 2026

Mercedes-Benz AMG GT 4-Door · 2019–2025

Community reported
220 owners
Link to this issueError Codes:P0A0FP0A1AP0B29P0CA7

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.
Typical repair cost:$1,200 - $3,000

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Based on 220 owner reports

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Community Reported
Medium Confidence220 owner reportsLast reported by owners Feb 2026Reviewed Mar 2026

Mercedes-Benz CLS-Class · 2019–2024

Link to this issueError Codes:P0A0FP0A1AP0B29P0CA7

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.
Typical repair cost:$1,200 - $6,500

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Community Reported
High Confidence

Audi A6 allroad · 2020–2024

Link to this issueError Codes:P0A7DP0B29

Audi TSB 2067906/5 explains that the 48V battery can enter low-state-of-charge protection and open its internal relays. P0A7D00 can accompany this condition. When measured 48V state of charge is 5%-15%, Audi directs a controlled recharge through the 12V electrical system and says the 48V battery must not be replaced for that condition. P0B2900 alone is also not a reason to replace the battery. This is a guided service procedure, not proof of battery degradation.

Common Symptoms

  • 48V battery state of charge at 5%-15%
  • 48V low-state-of-charge protection active

How to Fix

Have an Audi-trained technician measure both battery states of charge and follow the current guided procedure. For a 48V battery at 5%-15%, TSB 2067906/5 requires an approved high-capacity 12V charger and the ODIS charging-contactor test plan; it is not a consumer battery-maintainer procedure. Audi permits replacement only when the 48V state of charge is below 5% or guided fault finding directs replacement. Do not buy a 48V battery, 12V battery, charger, tape, wire loom or connector kit from this diagnostic card.

Sources for this issue (1)
  • Audi TSB 2067906/5 - 48V Battery Low-State-of-Charge Recharge Proceduretsb

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Manually Verified
High ConfidenceVerifiedReviewed Jul 2026

Find P0B29 by make

Browse every documented model and issue for your make.

Audi · 1 issueMercedes-Benz · 6 issues

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?

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.

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Content compiled with AI assistance using NHTSA complaints, TSBs, and owner reports. May contain errors. Always verify with your vehicle's service manual.

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