What to Do RIGHT NOW?
🚨 Immediate Actions to Prevent Further Issues 🚨
1️⃣ Unlatch the Frunk
- Your 12V battery is about to die.
- Once it does, you won’t be able to open the frunk electronically.
- Open it now using the MCU (that’s the touchscreen in the car: Main computer unit) or key fob while you still can.
2️⃣ Roll Down the Driver’s Window (if inside a garage)
- If the 12V battery dies, electronic door handles may stop working.
- Keeping a window open ensures you can access the car if needed.
3️⃣ Understand the Situation
- The Battery Management System (BMS) has detected a critical issue and opened the HV battery contactors.
- This cuts off power flow to and from the HV battery.
- The car won’t charge and won’t support the 12V battery, even though systems like the MCU and onboard computers are still drawing power.
- This will quickly drain the 12V battery.
4️⃣ What You’ll See on the MCU
- Expect multiple errors—sometimes dozens.
- Some will reference the 12V battery not being supported or needing replacement.
- The 12V battery itself isn’t the main problem—but letting it die will make troubleshooting much harder.
5️⃣ Protect the 12V Battery
- Disconnect the 12V battery if you’re not troubleshooting immediately.
- Connect it to an external charger to prevent complete discharge.
6️⃣ If the 12V Battery Dies Before You Open the Frunk
- You’ll need to manually open the frunk using the emergency release (Google for your specific Tesla model).
- This involves accessing the 12V jump terminals behind the front bumper or pull cords.
✅ Taking these steps now will save you a headache later. If the 12V dies, your troubleshooting options become much more difficult unless you have a 50A+ external 12V power supply (which you very likely don’t own).
What Just Happened?
Tesla’s Battery Management System (BMS) is responsible for monitoring and controlling the high-voltage (HV) battery. When a critical fault is detected, the BMS shuts down the system to prevent potential hazards.
- Tesla’s strict safety controls help keep fires and catastrophic failures extremely rare.
- A battery failure can be caused by many different issues, some minor and some severe.
- Using Scan My Tesla can help analyze the issue and determine the severity.
- An external isolation fault in the HV system (such as a failure in the drive unit, AC system, or heater) can also trigger a shutdown, even if the battery itself is fine.
- At first glance, symptoms look the same (12V issues, car won’t run or charge), but the specific errors in Service Mode will help distinguish battery issues from other failures.
⚠ Non-battery-related failures (such as issues with HV components outside the pack) require a different troubleshooting process, which is beyond the scope of this guide.
Managing the 12V System Without HV Support
Tesla’s 12V battery powers essential systems, but when the DC-DC converter is offline, it will drain quickly.
- A trickle charger or Battery Tender alone isn’t enough to keep up with the power draw.
- Think of this like an internal combustion engine (ICE) car running without an alternator—it may work briefly, but critical systems will fail quickly as the battery loses charge.
🔋 How to Keep the 12V Alive During Troubleshooting:
- Use a charger rated for at least 10A (preferably higher).
- Ideally, connect a second large 12V battery in parallel using jumper cables and then attach a heavy-duty charger to that.
- For most of the repair process, keep the 12V battery disconnected and periodically recharge it with a basic trickle charger.
- When diagnosing the issue in Service Mode or with Scan My Tesla, ensure the 12V battery is adequately supported to prevent it from dying mid-diagnosis.
🚨 A dead 12V battery makes troubleshooting harder—keeping it charged is important to diagnosing and fixing the HV issue further and keeping it healthy can save you having to replace the relatively expensive 12V battery as well.
Final Thoughts
If your Tesla has a critical HV fault, your top priority is preventing the 12V battery from dying. Follow these steps immediately to avoid further complications. Taking a few precautions now will save a lot of time and frustration later! 🚗⚡
Understanding Tesla’s Error Code Syntax
Tesla’s Battery Management System (BMS) monitors the HV system, including the High Voltage Interlock Loop (HVIL). Not all BMS errors pertain directly to the HV battery pack, but many do.
Tesla BMS error codes follow a structured format:
🔹 BMS_wXXX – Warning Code
- The “_w” means “warning.”
- This indicates a potential issue, but the system has not taken any major action yet.
- If not addressed, the problem could escalate, leading to further restrictions or shutdowns.
🔹 BMS_uXXX – User-Facing Code
- The “_u” means “user.”
- These are more serious than warning codes and will often appear on the screen for the driver.
- They may limit functionality but don’t always shut the car down immediately.
🔹 BMS_fXXX – Failure Code
- The “_f” means “failure.”
- These indicate a critical issue that has already caused a shutdown.
- When a failure occurs, the car typically disables the HV system to prevent further damage or safety risks.
Common HV Battery Faults – Understanding Tesla BMS Errors
Tesla’s Battery Management System (BMS) monitors the HV battery pack and ensures safe operation. When a fault is detected, the BMS may reduce performance, limit charging, or completely shut down the pack. Below are some of the most common HV battery errors, their causes, and what they mean.
BMS_u029 – Maximum Battery Charge Level Reduced
🔹 What it Means:
- The BMS has detected that the CAC (Calculated Average Capacity) of the bricks is no longer balanced within the expected range.
- This is a long-term calculation and does not reset on its own, even if the pack is repaired.
🔹 Symptoms:
- Reduced maximum charge level (i.e., car won’t charge to 100%).
- Persistent warning message.
🔹 Fix:
- The pack must be repaired, and the error manually cleared after balancing using Toolbox 3.
BMS_u018 – Short-Term Voltage Imbalance Between Bricks
🔹 What it Means:
- Similar to u029, but this error is based on short-term voltage variations rather than long-term CAC drift.
- The error clears on its own once the issue is resolved.
🔹 Symptoms:
- Contactors should still close, meaning the car can still charge and drive, though with reduced charging capacity and range.
- The error may disappear overnight if the pack is balanced.
🔹 Potential Causes:
- Temporary voltage drift between modules – (can be ok).
- A failing BMB (Battery Management Board) with deteriorating capacitors, causing intermittent voltage readings (needs addressing).
🔹 Fix:
- If the pack has been repaired, manually balancing the modules and allowing the system to sit overnight may resolve the issue.
- If caused by a failing BMB, the problem will return intermittently and get progressively worse.
BMS_f107 – High Voltage Sensor Error
🔹 What it Means:
- The BMS is unable to read voltage from two or more bricks.
- Can be caused by:
- A failed BMB board that no longer sends voltage data – see V1 pack issue.
- A broken HV sense wire inside the pack – see V2 pack issue.
🔹 Tesla’s Response:
- This was originally a latching fault, but Tesla has made it non-latching in v2 packs to allow cars to keep driving under certain conditions.
🔹 Symptoms:
- Car may shut down if the fault is severe.
- Sometimes the issue disappears momentarily if a loose sense wire regains contact, but it will return.
- Common after accidents, as Tesla’s “quadruple redundancy” sense wires can still break from impact.
🔹 Fix:
- If the error is caused by loose or corroded sense wires, these must be repaired or replaced.
- Scan My Tesla may still show all 96 voltage readings as “valid”, but the fault can be intermittent.
BMS_f123 – Internal Isolation Fault (Water Damage)
🔹 What it Means:
- Water has entered the pack, causing an internal isolation fault (high-voltage leakage to the metal casing).
- This can be intermittent as water moves around inside the pack.
🔹 Symptoms:
- The error may clear temporarily if the pack dries out, but it will return if moisture remains.
- If rust or corrosion starts forming, the pack can quickly become unusable.
🔹 Fix – No Shortcuts!
-
You CANNOT dry the pack by:
❌ Blowing air through the case
❌ Using rice, desiccant packs, or heat lamps
❌ Draining water from umbrella valves -
The only proper way to dry the pack is to open it completely and remove/dry each module individually.
🔹 Tesla’s “Tilt Maneuver” (For Rusted Fuse Covers)
- Tesla performs a temporary tilt maneuver to drain some water out, making the pack safer to handle and transport.
- This does not fix the issue—it only reduces immediate risk before opening the pack for repairs.
⚠ Packs with water intrusion are extremely dangerous! Always review safety protocols before working on a wet pack.
External Isolation Faults (HV Component Failures)
🔹 What it Means:
- Sometimes, HV isolation errors are NOT caused by the battery pack itself.
- A failure in a drive unit, HVAC system, or heater can trigger an external isolation fault, shutting down the pack.
🔹 Symptoms:
- Car won’t charge or drive, just like an internal isolation fault.
- However, the HV pack itself is fine—the issue is elsewhere.
🔹 Fix:
- Diagnosing non-battery HV isolation faults requires different troubleshooting methods.
- The NTSB has released a bulletin detailing external HV failures.
Final Thoughts
- Some BMS errors clear automatically, while others must be reset after repair.
- Scan My Tesla can provide useful voltage readings to help diagnose problems.
- Isolation faults require extreme caution, especially if caused by water intrusion.
- If in doubt, consult experienced professionals—high-voltage mistakes can be deadly.
🚗⚡ Stay safe and troubleshoot methodically!
