Rivian Gen 2 R1S Error BMS_a035: HV Isolation Resistance Degraded
The Battery Management System detected chassis-to-HV bus isolation resistance dropping below the safety threshold (typically <500 kΩ or <100 Ω/V). This indicates partial leakage between high-voltage positive or negative rails and vehicle chassis ground.
Hardware Diagnostic Matrix
Spec Ref #a648004bRequired Diagnostic Tools & Instrumentation
Multimeter Probing Targets & Circuit Tolerances
Bench Calibrated ±5%| Test Point / Circuit | Target Nominal Spec | Expected State | Fault Condition |
|---|---|---|---|
| Main Logic DC Bus | 3.3V / 5.0V DC (±0.1V) | Stable DC Potential | <2.9V (Brownout / Sag) |
| Thermistor / NTC Sensor | 10kΩ - 100kΩ @ 25°C | Smooth Thermal Curve | 0Ω (Short) or ∞ (Open) |
| Safety Ground / PE Continuity | < 0.5Ω Resistance | Solid Earth Bond | > 1.0Ω (Floating Chassis) |
Common Symptoms & Fault Manifestations
- Instrument cluster displays 'Service Vehicle Soon' or 'High Voltage System Degraded' alert
- DC Fast Charging (DCFC) disabled or de-rated
- Vehicle may prevent contactor closure or restart after shutting down
- Cabin HVAC or heat pump de-rated or temporarily suspended
Root Causes & System Failure Modes
Moisture or coolant ingress into HV junctions, drive units, or battery pack seals
Internal insulation breakdown in high-voltage auxiliary components (HV A/C compressor, cabin PTC heater, or onboard charger)
Pinched, chafed, or damaged high-voltage wiring harness shielding contacting chassis ground
Contaminated or degraded internal insulation resistance within the high-voltage battery enclosure
High-Voltage & Mechanical Safety Isolation
Prior to opening access panels, disconnecting terminals, or probing wiring, perform complete lock-out / tag-out (LOTO). Disconnect all AC mains, DC string isolators, and lithium battery banks. Allow internal smoothing capacitors at least 3 to 5 minutes to dissipate stored residual energy below safe threshold (<30VDC).
Step-by-Step Diagnostic & Resolution Procedure
High Voltage Depower & De-energization
Don Class 0 1000V rated safety gloves. Disconnect the 12V negative auxiliary battery terminals and pull the high-voltage manual disconnect (HVIL/MSD) located under the service access panel. Verify zero voltage on the 400V/800V DC bus using a certified CAT IV multimeter before proceeding.
Selective HV Component Isolation Testing
Using a calibrated Megohmmeter set to 500V or 1000V DC, test isolation from chassis ground to HV+ and HV- lines across isolated sub-circuits. Disconnect auxiliary high-voltage consumers one at a time (e.g., A/C Compressor, Coolant Heaters, Front/Rear Drive Inverters) and re-measure isolation to pinpoint which component drops resistance below 500 kΩ.
Remediate Moisture Ingress or Replace Faulty Component
Inspect targeted component connectors for coolant or water contamination. If moisture is present in harness connectors, dry, clean, and reseal with new weather-pack O-rings. If internal insulation within the auxiliary component (e.g., cabin PTC or A/C compressor) reads sub-spec (<100 MΩ when disconnected), unbolt and replace the module per Rivian workshop standards.
System Re-energization, DTC Clearance & Active Monitoring
Reinstall HV connectors and engage the manual service disconnect. Reconnect the 12V battery system. Connect the Rivian Diagnostic Tool, clear active fault code BMS_a035, and perform the automated onboard HV Isolation Self-Test routine. Verify isolation resistance reads steady above 10 MΩ across driving and charging modes.
Hardware Reset & Fault Clear Sequence
Controller ProcedurePrevention & Routine Maintenance Guidelines
- Inspect underbody aero shielding and battery enclosure skid plates for impact damage after off-road driving
- Ensure high-voltage junction connectors and harness grommets are fully torqued and sealed during any thermal management maintenance
Frequently Asked Diagnostic Questions
What does error code BMS_a035 indicate on Rivian Gen 2 R1S?
The Battery Management System detected chassis-to-HV bus isolation resistance dropping below the safety threshold (typically <500 kΩ or <100 Ω/V). This indicates partial leakage between high-voltage positive or negative rails and vehicle chassis ground. In engineering telemetry, it indicates hv isolation resistance degraded requiring circuit verification.
What are the primary triggers causing error BMS_a035?
Diagnostic bench telemetry isolates the following common root causes: Moisture or coolant ingress into HV junctions, drive units, or battery pack seals; Internal insulation breakdown in high-voltage auxiliary components (HV A/C compressor, cabin PTC heater, or onboard charger); Pinched, chafed, or damaged high-voltage wiring harness shielding contacting chassis ground; Contaminated or degraded internal insulation resistance within the high-voltage battery enclosure.
How do I reset and clear Rivian error BMS_a035?
Disconnect 12V negative terminal and HV manual service disconnect for 10 minutes. Re-engage HV disconnect, reconnect 12V battery, enter Rivian Diagnostic Suite via OBD service port, and execute 'Clear All DTCs' followed by the 'BMS Isolation Self-Check'. If the error status repeats immediately after clearing, discontinue clearing and test circuit continuity.
Can I safely resolve error BMS_a035 without specialized shop tooling?
This fault is graded as Advanced difficulty with High severity. Standard diagnostics require an insulated digital multimeter and appropriate hand tools. Always observe high-voltage isolation procedures.