Tesla battery errors (BMS): Service Mode codes decoded, what they mean & repair cost (2026)

Tesla hides the specifics from the owner, but the codes in Service Mode name the problem precisely. This article is a single breakdown of the "scary" battery codes: what each one means, who it affects (model, year, chemistry) and what it costs in Belarus, with links to the detailed write-ups. People google these codes character-for-character, so they are spelled out in full.

How to pull the codes: Service Mode (Controls → Software → hold the model name ~2 s → code "service") → the Alerts section. Format: BMS_a066_SOC_Imbalance_Warning. Decode via stats.tessie.com and tesware.net.

Summary: code → case → price

1. Module imbalance: BMS_a066 → a064 → a079

The most common "scary" scenario. Escalating in stages:

• BMS_a066_SOC_Imbalance_Warning: early warning, "Maximum charge level and range may be reduced". You can keep driving.
• BMS_a064: the BMS caps the maximum SOC.
• BMS_a079: limits charge (often to ~50%); a mass issue on Model 3/Y (often paired with BMS_a029).

Affects NCA Panasonic on 2170L cells: US Model 3/Y Long Range 82 kWh (2021-2022 refresh) and Standard Range 55 kWh from 2021 ("Panasonic 1L"). This is the same problematic L-chemistry as the 82, and the SR-55 also drifts into imbalance often, but it was built for only a short time (essentially Q1 2021 before the switch to LFP), so donor modules are noticeably harder to find at salvage yards than for the 82.

In Belarus $1,500-7,000 (module/pack replacement; depends on donor availability). More detail: 82 kWh Panasonic imbalance · module/cell replacement · 54/75 pre-refresh packs.

2. Voltage sense tab failure: BMS_f107

• Codes: BMS_w107 / BMS_f107 and related voltage-measurement circuit codes.

• Thin contact "tabs"/harnesses (Voltage Sense Harness) read voltage off the cell groups; vibration and thermal cycling tear the contact loose → the BMS loses its readings → pack shutdown.

• Affects S/X 100 kWh (V2 packs 2016-2020). A close relative: module contact "tabs" coming loose on the pre-refresh M3/Y 2017-2020.

• tsk.by field experience ("error 107" on the 100 kWh Long Range / LR Plus): the shop traces the voltage sensor failures to an incorrectly configured robot that soldered the wiring inside the battery at the factory. So fixing a single module doesn't guarantee the result: the defect is factory-made and common to the whole pack, and the complete fix is upgrading the sensors of all modules (tsk.by breakdown).

• In Belarus $1,000-3,000 (more often ~$2,000).

• More detail: Voltage Sense Harness tabs on the S/X 100.

3. Moisture in the pack and isolation: BMS_a035, BMS_a037

• BMS_a035 / BMS_w035: isolation alerts (often a sign of moisture in the pack).
• BMS_a037_SW_Flood_Port_Open: the battery's drain (flood) port is open.
• Affects the old S/X 85 (water enters through the breathing valves); the flood port affects M3/Y and S/X 2021+. The refresh Palladium also has a vulnerable floor.
• More detail: moisture in the 85 via valves · Palladium 100: vulnerabilities.

4. Stuck high-voltage battery contactor: BMS_f038

• Codes: BMS_f038 / BMS_w026 / BMS_w086.
• The power contactor (on classic S/X, in the "penthouse" inside the pack) sticks or won't open. In Service Mode → Battery the bus voltages should drop to zero within 30-60 s of the disconnect command; if they don't, the contactor is stuck.
• More detail: stuck HV battery contactor.

5. A "battery" error that is really about the drive unit: BMS_w172

• BMS_w172_SW_Drive_Iso_Warning (often paired with BMS_u008): an isolation error. On S/X 2012-2020 the cause is almost always a coolant leak through the rotor seal of the rear motor (LDU), not the battery itself.
• More detail: rear drive unit problems on S/X.

6. Long-term and "non-repairable" chemistries (no single catastrophe code)

• LG NCM (M3/Y LR refresh): no single code, instead rising internal resistance and individual cell failures by ~240k km; modules are often not economically repairable. The nuance and the counter-argument are in the 79 kWh LG "grail" article.
• 4680 (Model Y Texas, Cybertruck): structural pack, inseparable from the body → any cell problem = full pack replacement, $15,000-25,000. See 4680 problems in Texas.
• Capacity drop after service ("HVC replaced") happens; measure SOH before and after.
• 12V/16V "brick": the traction battery is fine, but the car "dies" entirely; fixed by an AGM/Li swap (12V battery brick).

7. HVIL loop break: the HV system won't power up at all

• Every high-voltage connector is wrapped in an HVIL (High Voltage Interlock Loop) connection-monitoring circuit. Road de-icing chemicals destroy these loops and their contacts over time. When the HVIL is broken, the BMS refuses to close the contactors: the high-voltage system won't power up, and the car goes to the shop on a flatbed (tsk.by breakdown).
• Affects any model driven on winter roads treated with de-icing agents, which means practically all use in Belarus.

What to do with any code

1. Pull the full list of codes in Service Mode → Alerts (and the history).
2. Connect Scan My Tesla: SOH, per-group imbalance (normal 2-6 mV, alarm from ~200 mV), temperatures.
3. Run the Battery Test in Service Mode: it is a diagnostic and gauge calibration, not a cure, because the BMS balancer already runs constantly on its own. If the code persists, the only question is the scale of the replacement.
4. Cross-check the pack and the risks against the battery chemistry guide and the used-buying checklist.

Clearing errors: when it's enough, and when the code will come back

Clearing codes by itself fixes nothing; it just wipes the records.

• Clearing is enough if the code is a historical trace of an already-solved problem: after component replacement, a brief glitch in a circuit, a 12V battery disconnect or a software update.
• The code will come back if the cause is real: a dying sensor, damaged wiring, cell degradation. The error reappears within minutes or kilometers, and clearing it is pointless until the cause is fixed.
• Clearing does not touch battery calibrations and data: the capacity/SOH statistics accumulated by the BMS stay in place.

Source: tsk.by