Model S 85 kWh and pre-facelift X: sticking umbrella valves and moisture in the battery

Typical picture

• Moisture or condensate gets into the HV battery housing through the umbrella valves (the official term in Tesla's service manual) or through a rusted metal fuse block cover.
• Corrosion starts inside, on the copper/brass busbars and connectors.
• In the worst case this ends in a short circuit and a fire (t.me/teslacar_chat/265953: "the brass oxidised → short circuit").
• The error in the logs: BMS_a037_SW_Flood_Port_Open (t.me/teslacar_chat/37193). It is the event code for a condensate drain valve that is stuck or leaking, and it is the one owners actually see. It often comes together with BMS_w035_SW_Isolation.

📝 Forums sometimes mention BMS_F107 (SW_Cell_Voltage_Sensor). That is a cell voltage sensor fault, but on S/X before 2020 it is often a consequence of the same moisture and corrosion. In current Service Mode you see BMS_a037.

What to check

• The umbrella valves on both sides of the pack. On S/X they sit along the bottom near the sills. A car put on a lift the wrong way can have them deformed by the jack, so check the lift points.
• Corrosion of the fuse cover. On older cars the metal cover rusts through within 10 years, and in winter water gets into the compartment through it (t.me/teslacar_chat/518930). It is replaced with a plastic one for ~$500.
• A housing seal test: about 0.19 psi according to the service manual. (This figure comes from the manual; owners in the RTC chat have not confirmed it directly.)

Repair

• Local leak repair, cleaning and drying, replacement of the affected BMBs/modules.
• With deep corrosion, the pack comes out and is repaired on the bench.

A confirmed case from EV Clinic Berlin (July 2025): a Model S owner had the battery serviced. "They dropped the pack → cleaned it → replaced the umbrella valves → sealed it back up → 1450 EUR (~$1550)" (t.me/teslacar_chat/682998). Use it as a market price reference for the full job in Europe.

Where exactly the water gets in (tsk.by field experience)

• The steel battery cover rots through from road de-icing chemicals. On Model S cars built before 2015 the pyro-fuse compartment cover rots especially often: water gets into the battery through it, and the car is immobilized with an isolation error (tsk.by breakdown).
• Condensate inside the housing corrodes contacts and lowers the insulation resistance, with the same result: an isolation error on the screen. The fix is drying with special equipment, removing the corrosion and restoring the housing's sealing (tsk.by breakdown).
• Repairability by generation: Model S batteries from 2012–2021 are repairable; from 2021 on (Palladium) they are conditionally repairable.

S/X from 2021 on: similar rumours

The RTC chat regularly gets questions about the same defect on the refresh S/X (Plaid/Palladium 100 kWh): t.me/teslacar_chat/603425, t.me/teslacar_chat/505060. There are no confirmed repairs on the refresh models yet. See our separate article on the refresh battery valves for details.

A telling detail: on the Cybertruck Tesla added active pressurisation of the battery housing with positive pressure (t.me/teslacar_chat/565042). That is an engineering admission that the breather valve problem is real, and Tesla is solving it on its new models.

Cost in Belarus

• Fuse cover only (metal → plastic plus cleaning): ≈ $500 (t.me/teslacar_chat/518930).
• Full valve repair (pack removal, cleaning, new umbrella valves, resealing): ≈ $1 500 (based on the EV Clinic Berlin case).
• With busbar/BMB corrosion: up to $4 000, depending on the extent of the damage.

Links

• Problem overview (valves/AC drain): https://www.autoevolution.com/news/check-the-tesla-model-s-moisture-ingress-problem-with-comprehensive-video-199337.html
• Video: cover corrosion and water in the fuse compartment: https://www.youtube.com/watch?v=GZN8pawcQkg
• RTC: EV Clinic Berlin, 1450 EUR for the full job: https://t.me/teslacar_chat/682998
• RTC: cover repair for $500 (David, March 2024): https://t.me/teslacar_chat/518930
• RTC: BMS_a037_SW_Flood_Port_Open, the real code in the logs: https://t.me/teslacar_chat/37193
• RTC: corrosion → fire (IC, December 2022): https://t.me/teslacar_chat/265953
• RTC: the situation on 2021/2022 S and X: https://t.me/teslacar_chat/603425