Unable to Charge — HV Isolation Fault Detected While Parked
The TESLA-BMS_A123 (Tesla BMS (Battery Management System)) EV fault code means: Unable to Charge — HV Isolation Fault Detected While Parked. This is a critical severity code.
- Keep driving?
- Short distances only
- DIY difficulty
- expert
- Estimated cost
- Diagnosed and quoted by Tesla service; a battery-side isolation fault can involve significant pack work. Battery-related repairs may be covered by the battery and drive unit warranty — check your coverage.
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Common Symptoms
- Alert on the touchscreen: "Unable to charge — Service is required"
- Vehicle refuses to charge
- Vehicle may be unable to drive or support other vehicle systems
- Detected while the vehicle was parked (battery contactors open)
Probable Causes (Ranked by Likelihood)
- Reduced electrical isolation detected in the high-voltage battery system while parked, with the battery contactors open (Tesla's official description); the vehicle limits functions when isolation falls below the acceptable level Likely
- Firmware-level alert name (BMS_a123_SW_Internal_Isolation, per the firmware alert dictionary) indicates isolation measured too low INSIDE the HV battery pack — the dictionary also notes a failed voltage-sense harness as a documented trigger Possible
Step-by-Step Diagnostic Procedure
This isolation fault was detected while the vehicle was parked with the battery contactors open — meaning the issue is on the battery side of the contactors.
Per Tesla's official guidance: do NOT attempt to charge the vehicle. It may be unable to charge, drive, or support other systems. Schedule service if the alert persists.
Tesla's safety escalation applies here: if you notice any unusual smells, sounds, or visible damage near the battery area, stop using the vehicle immediately and contact Roadside Assistance.
High-voltage isolation diagnosis is Tesla-service-only work — never attempt to inspect or repair the high-voltage battery yourself.
Frequently Asked Questions
What does BMS_a123 mean on a Tesla?
Tesla's official meaning: the vehicle detected reduced electrical isolation in the high-voltage battery system while parked, with the battery contactors open. High-voltage batteries are electrically isolated from the vehicle frame for safety, and the vehicle limits functions when that isolation drops below the acceptable level.
Can I charge my Tesla with BMS_a123?
No — Tesla's official guidance is explicit: do not attempt to charge the vehicle. Schedule service, and if you notice unusual smells, sounds, or visible damage near the battery area, stop using the vehicle immediately and contact Roadside Assistance.
Can I drive with BMS_a123?
The vehicle may be unable to drive at all. If it does drive, limit it to what's necessary to get the vehicle somewhere serviceable, and stop using it immediately if you notice anything unusual near the battery area.
Sources
This page is built from documented references. Verify against your own service info before repair work.
- Tesla Model 3 Owner's Manual — Troubleshooting Alerts (official Tesla alert text for BMS_a123)
- Tesla Model Y Owner's Manual — Troubleshooting Alerts (confirms BMS_a123 is documented across models)
- Tesla firmware alert reference (core-hacked/tesla-alerts-dictionary): bms_a123
- Tessie Tesla alert statistics