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

Code by code. Full alert names as they appear in Service Mode, meanings from the Tessie database. Every row has an anchor you can link to, for example #bms_a079. A price is given only where the chat has a case; group prices are in the table above.

1. Module imbalance: BMS_a066 → a064 → a079

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

• BMS_a066_SOC_Imbalance_Warning: early warning with a lower threshold than a064; there is no on-screen message, the code shows only in Service Mode. You can keep driving.
• BMS_a064_SW_SOC_Imbalance: the BMS caps the maximum SOC (at 50%).
• BMS_a079_Max_Charge_Level_Exceeded: "Unable to charge - Maximum charge level reached", the charge is above an already lowered limit (usually ~50%; the chat has also seen 32%, #298175); a mass issue on Model 3/Y (often paired with BMS_a029). The screen then shows BMS_a074_Max_Charge_Level_Reduced, "Maximum battery charge level reduced".

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.

Imbalance codes also show up outside this chemistry: the chat has an EU Model Y Performance 2023 with BMS_a066 (pack replaced under warranty in Berlin, #293967) and a Model 3 2019 Standard with BMS_a079 at 140,000 km (#404023). For scale: in South Korea, 4,351 cars have logged BMS_a079 since 2017 (4,637 incidents), according to Korea JoongAng Daily data (Drive Tesla Canada, October 2025).

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.
• A reset won't help: the car re-tests isolation every time it powers up, and the error clears by itself only once isolation is actually restored (#472837, #472850). Without Toolbox, Service Mode shows only the external isolation reading (#420470).
• 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.
• BMS_f086_SW_Not_Ok_To_Precharge / BMS_w086_SW_Not_Ok_To_Precharge: the opposite case, the BMS blocks precharge and the contactors won't close. BMS_w026_SW_Ctrs_Disabled: contactors disabled until service; per Tessie this alert can only be cleared with Toolbox 3. In the chat, the f086 + w026 set hit a Model S after a fault at a DC station in Minsk (#55431) and a Model X 2019 after a hit to the rear wheel (#10437). On a Model S the lockout was cleared with a Toolbox reset (#55445), but the cause has to be found first.
• 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.

Other BMS codes

Codes people often search for that the sections above don't cover. Only verified definitions are used here (the Tessie database and Tesla's Service Mode guide); causes and prices that the sources don't give are left out.

BMS_a117_SW_Delta_SOC_Weak_Short

• What it means: one brick (a parallel group of cells) repeatedly shows an alpha estimate below the threshold (Tessie). The name reads as "delta SOC, weak short": the BMS sees the group's charge drifting away from the rest and suspects a weak internal short, in other words elevated self-discharge.
• How it shows up: there is no on-screen message, the alert is visible only in Service Mode Plus (audience: service). According to Tessie it doesn't affect how the car works on its own, it is a log entry; it is cleared not by a reset but by a service routine (UDS routine CLEAR_SOC1_SNAPSHOT).
• Related codes: on classic S/X the same algorithm is called BMS_w117 / BMS_f117, and with the dSoc alert enabled it sets BMS_u029_dSoc_Limiting, a charge limit at about 50% SOC. On Model 3/Y the equivalent limit is BMS_a029_dSoc_Limiting, and the driver sees BMS_a074.
• What to do: track the voltage spread in Scan My Tesla instead of waiting for the limit to kick in. The mechanism is the same as the current leaks in the 82 kWh Panasonic pack: the faulty group discharges faster than its neighbors. The chat has no a117 cases yet.

BMS_a170_SW_Limp_Mode

• What it means: the BMS has reduced drive power; the screen shows "Acceleration and top speed reduced - Performance may be restored on next drive" (Tessie). Model 3/Y and S/X 2021+ platform; on classic S/X the equivalent is BMS_u008_limpMode.
• What the chat saw: the "turtle" and a170 right after an overnight charge, gone after a few minutes of driving and a short stop (#139221); a Model 3 2019 with a170 that would not engage Drive at all (#223564); a Model 3 Performance 2022 where a170 kept coming back together with BMS_a088 (#405031).
• Key points from the replies: a170 is a consequence, not a diagnosis; the neighboring alerts show the cause (#223611). If one motor fails, a car with a170 may still drive; if CAN communication is lost, it won't. In the case discussed, the car could not see the front motor (#223617, #223619). Another experience: a170 appeared after charging and went away again, and a pyro fuse replacement cured it (#223604); see HV battery pyro fuse.
• Paired with BMS_a088: BMS_a088_SW_VcFront_MIA_InDrive means the BMS is not receiving messages from the VCFRONT front controller while driving; the screen shows "Vehicle may not restart" (Tessie).
• What to do: open Recent Alerts and see what fired in the same second as a170.

BMS_a036_SW_HvpHvilFault

• What it means: the BMS sees the HVIL loop shorted while the contactors are closed (HVIL voltage lower than expected); the screen shows "Vehicle may not restart - Service is required" (Tessie). Model 3/Y and S/X 2021+ platform.
• HVP link: a fault in the same loop seen by the high-voltage controller sets HVP_w026_HvilFault: until the HVIL is intact, the contactors won't close and the car won't drive; if the alert cleared by itself without a repair, it may come back (Tessie).
• What to do: HVIL is the loop from section 7; when it is broken, the contactors won't engage (#137). The usual search is for a bad contact in the high-voltage connectors; the chat has no separate a036 cases.

BMS_a255, BMS_a709, BMS_d265: no public definition

• The public Tessie database (about 17,000 alerts across all modules, checked on 2026-10-20) has no such BMS alerts, and they have never come up in the chat. BMS alerts with the letter a end at a181 there, and those with d end at d014.
• A number without its prefix means nothing. a255 exists on other modules, for example VCFRONT_a255_pumpPtStopped: the powertrain coolant pump has stopped (below 800 rpm while flow is requested), and power may be limited (Tessie). So first check that the screen really says BMS_.
• The car itself gives the explanation: in Service Mode the alert name follows the number, and tapping the alert expands the details. Owners in the chat quote Potential Impact, Set condition and Alert Payload (signal values at the moment the alert fired) from there (#143078, #42057).

How to read a Tesla alert code: prefix → module

Where to look. Service Mode (Controls → Software → hold the MODEL text ~2 s → code "service") → Service Alerts. Active Alerts shows what is active now, Recent Alerts the last 100 alerts by time; there are Customer / Service-Fix / All filters, and the Manual button opens the Troubleshooting Alerts section of the owner's manual (Tesla's Service Mode guide).

Who the alert is for. Every alert has an audience: customer means the driver sees a two-line message on screen; service-fix means a repair is needed and the alert shows only in Service Mode; service is reference information visible only in Service Mode Plus.

How the name is built. BMS_a066_SOC_Imbalance_Warning = module (BMS) + letter and number (a066) + name (SOC_Imbalance_Warning). A number is unique only within its module: BMS_a036 is HVIL, while DIR_a036_busOV is high-voltage bus overvoltage at the rear inverter; BMS_a117 is a weak short in the battery, while CP_a117_inletHeaterUnableToRegulate is the charge port inlet heater. Even one code can mean different things on different platforms: GTW_w019_bmsMia on classic S/X and GTW_w019_CPversionMismatch on newer cars. So search for the full name, not the number.

Letters. In the Tessie database (checked for every code in this article), BMS alerts with the letter a belong to the Model 3/Y and S/X 2021+ platform, while w, f and u belong to classic S/X 2012–2020. On the classic cars one number often exists as a w/f pair with a different vehicle response (BMS_w107: "Vehicle may not restart", BMS_f107: "Service is required"), and the u codes u008, u018 (SOC imbalance, 50% charge limit) and u029 are shown to the driver. Cybertruck battery alerts use other prefixes: for example, HVREX_a259_MaxChargeLevelExceeded is the counterpart of BMS_a079.

Timing matters more than count. Several alerts that fired in the same second are one event, not several failures: five THC alerts at once on a Model X 2019 were read in the chat as a single fault, most likely an empty refrigerant circuit (#434523).

What to do with any code

1. Pull the full list of codes in Service Mode → Alerts (and the history).
2. Check which alerts fired in the same second: one event often produces a batch of codes with a single cause (#434523).
3. Connect Scan My Tesla: SOH, per-group imbalance (normal 2-6 mV, alarm from ~200 mV), temperatures.
4. 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.
5. 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