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

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.

How the leak is found

1. Measuring the isolation resistance with a megohmmeter between the HV bus and the body.
2. Sequentially disconnecting sections of the HV circuit — until the unit "dragging down" the isolation is found.
3. Analyzing the onboard monitoring data (when and under what conditions the resistance dropped).
4. Fixing the cause: drying, cleaning corrosion, replacing the damaged cable/connector, resealing.
5. 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.

Links

• Tesla isolation diagnostics — tsk.by: https://tsk.by/uslugi/diagnostika-izolyatsii/