Theory and operation
Isolation monitors estimate resistance from the HV bus to chassis; positive-side and negative-side faults can shift reported bias differently by design.
Battery & High Voltage / Diagnostic guide
ISOLATION-FAULT DECISION GUIDE.
Use scan data and OEM segmentation to locate leakage toward chassis without unsafe exploratory probing.
EV GUIDE08/24
Use this guide to organize evidence. Vehicle-specific high-voltage access, measurements and component isolation require OEM procedures, appropriate qualifications, rated PPE and approved test equipment.
GUIDE PURPOSE
Diagnose the system—not one isolated value.
Confirm vehicle state, operating conditions and the full DTC record before clearing evidence or opening the high-voltage system.
Diagnostic evidence
Record isolation value, polarity clues, humidity, coolant exposure, charging state and which contactors or components are connected when the value changes.
Safe diagnostic path
Follow the OEM isolation tree and approved disconnect sequence to segment circuits only after proper disable and absence-of-voltage verification.
Boundary and common error
Never megohm-test connected power electronics or disconnect HV components to hunt a fault without the exact OEM procedure.
Specifications, connector access, disable sequences, wait times and allowable tests vary by model and voltage architecture.