2022 Mercedes-Benz EQS 450+

Mercedes EQS P0AA6 HV Isolation Fault — HV Distribution Block Seal Failure

P0AA6 B105987 Published 2025-02-24 Updated 2025-02-24
Mercedes EQS P0AA6 HV isolation distribution block seal water ingress

2022 Mercedes EQS 450+ presented with P0AA6 after heavy rain. HV negative bus isolation 19 kΩ. Failed seal on the underhood HV distribution block allowed water accumulation inside the block, creating a leakage path from HV negative to chassis.

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The Problem
2022 Mercedes-Benz EQS 450+ (34,800 miles) arrived with 'Electric Vehicle System Fault' and complete powertrain lockout following a day of heavy rainfall. P0AA6 and B105987 stored. Customer noted a brief 'EV System' warning the previous evening that had cleared.
Diagnostic Procedure
  1. 1. Confirm P0AA6 and B105987. Negative bus isolation: 19 kΩ. Positive bus: >1 MΩ.
  2. 2. Negative bus fault. Segment: disconnect negative bus at HV distribution block (underhood, passenger side). Isolation jumps to >1.5 MΩ. Fault contained to distribution block or its downstream connections.
  3. 3. Open HV distribution block cover. Water visible inside, pooled around the HV negative busbar. Mineral deposits indicate this has happened before (previous brief fault cleared as water dried).
  4. 4. Inspect block cover seal. Perimeter seal has a 60mm section that has delaminated from the cover — a known issue on 2021-2022 EQS vehicles covered under Mercedes Technical Bulletin LI 54.10-P-058629.
  5. 5. Dry block thoroughly. Replace distribution block cover seal per TB LI 54.10-P-058629 (seal kit available separately — no need to replace entire block).
  6. 6. Verify no corrosion on busbars. Surface clean — early intervention prevented corrosion damage.
  7. 7. HV isolation: negative >2 MΩ. Clear P0AA6. Vehicle enters Ready.
  8. 8. Test drive 20 miles. No fault return. Customer informed of TB and advised against pressure washing engine bay.
Resolution & Root Cause
HV distribution block dried and cover seal replaced per TB LI 54.10-P-058629. Isolation restored. P0AA6 resolved.
💡 Key Lesson
Mercedes EQS P0AA6 after rain — check for TB LI 54.10-P-058629 before any HV diagnosis. The underhood HV distribution block cover seal delamination on 2021-2022 EQS is a documented issue with a standalone seal kit repair. The fault is intermittent (clears when water dries) but recurs with every rain exposure. A $45 seal kit versus a misdiagnosed HV component replacement.
About This Case

This case was solved remotely by an HVDesk specialist with 15+ years of hands-on experience across major EV platforms including Tesla, Hyundai/Kia, Volkswagen ID series, BMW i-series, and Ford EVs. The procedure was provided as structured remote support to an independent auto repair shop.