High-voltage isolation fault: where it comes from and how the leak is found

Quick FAQ

What does an isolation fault mean on a Tesla?

The onboard monitoring system has detected current leaking from the high-voltage circuit (350–800 V depending on the model) to the body. It's not a 'glitch': the isolation is compromised somewhere: in the battery, the power cables, the inverter, the motor, the charger or the climate system. The car may limit power and speed, block charging, and in serious cases block driving entirely.

Why does an isolation leak occur?

Typical causes: mechanical damage to the HV cables, moisture in connectors after deep puddles or pressure washing, insulation aging, contact corrosion from road de-icing salt, and the aftermath of an accident. In the Belarusian climate with winter road chemicals the problem shows up noticeably more often.

Can I keep driving with an isolation fault?

Better not. Even if the car still drives, current leaking to the body is a matter of electrical safety and progressive damage: corrosion and moisture don't stop on their own. Once the leak crosses the threshold, the BMS opens the contactors and the car stops, possibly in an inconvenient place. Get it diagnosed without delay.

How does a shop find the leak location?

The method: measure the isolation resistance with a megohmmeter between the high-voltage bus and the body, then disconnect sections of the HV circuit one by one until the leaking unit is localized, plus analyze the onboard monitoring data. After the repair, a control re-measurement. Minsk reference prices: diagnostics from 150 BYN, fixing the leak from 200 BYN.

How is an isolation fault different from a broken HVIL?

They are different protections. Isolation is about current leaking from the HV circuit to the body (resistance dropped below the threshold). HVIL is a control loop confirming the physical connection of the HV connectors: if the loop is broken, the BMS won't close the contactors at all, and the HV system never powers up. The symptoms look alike (the car won't drive), but the causes and repairs are different.

How do I reduce the risk of isolation problems?

Check the isolation resistance at every scheduled service, don't storm deep puddles, wash the underbody periodically in winter to remove road salt, and keep an eye on the seals and protective covers of the HV cables. After a hard hit to the underbody or an accident, an inspection is mandatory.

What this system is

The high-voltage part of a Tesla runs at 350–800 V depending on the model and battery configuration. Isolation separates everything power-related from the body: the battery pack, HV cables, inverter, motor, charger and the climate system with its heat pump. The onboard isolation monitoring continuously measures leakage: as soon as the resistance drops below the threshold, an isolation fault appears on the display (tsk.by breakdown).

In Service Mode codes this is most often the BMS_a035 / BMS_w035 family; more in the BMS code hub.

Where the leak comes from

Mechanical damage to the HV cables (rocks, a lift, careless repairs).
Moisture in connectors after deep puddles or pressure washing.
Insulation aging on older cars.
Contact corrosion from road de-icing salt: the Belarusian winter makes this one of the main causes.
Accident aftermath, including hits to the underbody.

A separate classic scenario for the old S/X is moisture inside the pack itself: see moisture in the 85 kWh batteries via the valves and rotten covers.

What it threatens

In escalating order: reduced range and slow charging → power and top-speed limiting → charging lockout → full no-drive lockout (the BMS opens the contactors). Which stage it reaches depends on the size and dynamics of the leak.

An isolation fault cannot be "cleared"

Shops get this request regularly: "I'll drop by, clear the isolation fault for me for $100". It does not work that way: an isolation fault is reset by nothing except a repair. On every power-up the car re-runs a full test of the entire high-voltage side rather than reading a stored fault memory, so a cleared code comes back on that same power-up cycle. If the leak really has been fixed, the fault goes away on its own, with no visit "for a reset" (#472816, #472836).

Looking for someone who will "clear it cheap" wastes time: in two days alone, two Model S 2015 70D owners came in with that request (#472837). The real repair range per chat estimates (2026) is $400–2,000, and it grows the longer people keep driving with the fault: moisture and corrosion do not stop by themselves (#472850).

How the leak is found

Measuring the isolation resistance with a megohmmeter between the HV bus and the body.
Sequentially disconnecting sections of the HV circuit until the unit "dragging down" the isolation is found.
Analyzing the onboard monitoring data (when and under what conditions the resistance dropped).
Fixing the cause: drying, cleaning corrosion, replacing the damaged cable/connector, resealing.
A control re-measurement after the repair.

Minsk reference, BYN (one shop's numbers, not the market): isolation diagnostics from 150 BYN, fixing the leak from 200 BYN.

Not to be confused with HVIL

HVIL (High Voltage Interlock Loop) is a different protection: a control loop confirming that all HV connectors are physically connected. If the loop breaks (the same road salt is a frequent culprit), the BMS won't let the contactors close at all. The symptom looks the same (the car won't drive), but the diagnostics and repair are different. Both topics are covered in the BMS code hub.

Prevention

Isolation measurement at every scheduled service (see maintenance in Belarus).
Don't storm deep puddles: for an EV, water is a hidden and delayed risk.
In winter, periodically wash the underbody to remove road salt.
Watch the seals and protective covers of the HV cables.
After a hit to the underbody or an accident, an isolation check is mandatory.

Sources

tsk.by
https://tsk.by/uslugi/diagnostika-izolyatsii/

Reviewed for accuracy by tsk.by · 26 July 2026

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