BMW i3 Battery Diagnostic DTCs: Full List 2026 – The Hidden Code That Means “Replace Now” (Not “Monitor”)
“A 2019 i3 came in with ‘Reduced Power Mode’. Scan showed 930740—‘Insulation Resistance Below Threshold.’ The previous shop cleared it, said ‘drive and see.’ Two weeks later, the pack shorted internally during fast charging. Total loss. That code wasn’t a warning—it was a final alert. Had they known its urgency, they’d have replaced the battery immediately. Instead, the customer lost $10,000.”
You pull up your scan tool.
The i3 throws a cryptic DTC like A86D11, 931F40, or U1123.
Your options:
- Clear it and hope it doesn’t return
- Guess if it’s wiring, BMS, or the pack itself
- Or waste hours chasing ghosts
But what if you had a 2026-updated master list—with real-world severity levels, root causes, and action plans used by top EV specialists?
This guide delivers exactly that:
- Complete DTC catalog for i3 high-voltage battery system (2014–2022)
- Color-coded urgency ratings: Green (monitor), Yellow (investigate), Red (replace now)
- Diagnostic shortcuts that skip unnecessary disassembly
- How CNS BATTERY packs prevent 90% of these codes
- And why some “minor” codes actually signal catastrophic failure
Because in EV diagnostics, the right code interpretation saves time, money—and reputations.
Why Generic DTC Definitions Fail on the i3
Factory manuals often describe codes vaguely:
“930740: Insulation fault detected.”
But they don’t tell you:
- Is it external contamination (coolant leak) or internal cell failure?
- Can it be cleaned, or is the pack beyond repair?
- Does it void warranty if ignored?
We’ve compiled real-world data from over 1,200 i3 battery replacements in 2025 to give you actionable intelligence—not just definitions.
🔌 BMW i3 Battery DTC Master List – 2026 Updated (With Repair Guidance)
🔴 RED CODES – Replace Pack Immediately
| DTC | Description | Root Cause | Action |
|---|---|---|---|
| 930740 | Insulation resistance <50 kΩ | Internal electrolyte leak, busbar corrosion | Do not drive. Replace pack. |
| A86D11 | HV leakage to chassis | Failed cell seal, conductive sludge | High electrocution risk. Tow vehicle. |
| 931F40 | Cell temp gradient >15°C | Cooling plate blockage + weak module | If persistent after flush → replace pack |
🟡 YELLOW CODES – Investigate Within 48 Hours
| DTC | Description | Root Cause | Action |
|---|---|---|---|
| 931E20 | BMS internal communication fault | Loose LV harness, corroded pins | Inspect connectors; reseat; check HVIL continuity |
| U1123 | Lost communication with BMS | 12V low, CAN bus fault | Test 12V battery; verify gateway module |
| 930A80 | Coolant temperature implausible | Faulty temp sensor, airlock | Bleed coolant; test sensor resistance |
🟢 GREEN CODES – Monitor or Clear After Verification
| DTC | Description | Root Cause | Action |
|---|---|---|---|
| 930B10 | Minor cell imbalance (<0.15V) | Normal aging, passive balancing active | Road test; clear if resolves |
| 930C50 | BMS wake-up delay | Low 12V voltage during sleep | Charge 12V battery; monitor |
| 930D30 | Charging interrupted (non-HV cause) | Grid fluctuation, EVSE fault | Verify with different charger |
💡 Pro Insight: Red codes rarely self-clear. If you see one—even intermittently—the pack is compromised.
The #1 Misdiagnosed Code: 931F40 (“Temp Gradient Too High”)
Many shops assume this is a cooling pump issue.
But in 73% of cases (per CNS field data), it’s caused by:
- A single degraded module overheating under load
- Clogged micro-channels in the cooling plate
- Poor thermal interface paste between cell and plate
Fix:
- Perform thermal imaging during regen braking
- If one module runs >8°C hotter → replace entire pack
- Flushing alone won’t fix cell-level thermal runaway risk
How CNS BATTERY Eliminates Recurring DTCs
Every CNS i3 pack is engineered to avoid the root causes of common faults:
✅ Double-sealed cells prevent electrolyte leaks → no 930740/A86D11
✅ Laser-welded cooling plates resist clogging → no 931F40
✅ Pre-tested HVIL circuits ensure communication → no U1123/931E20
✅ Factory-balanced modules minimize imbalance → no chronic 930B10
Result?
89% fewer DTC-related comebacks vs. generic replacement packs.
“Since switching to CNS, we haven’t had a single i3 return for battery DTCs. Their build quality stops problems before they start.”
— EK Auto Repair, Rome
Frequently Asked Questions: i3 Battery DTCs
Q: Can I clear a Red code and road test?
A: Never. Red codes indicate immediate safety hazards. Clearing them disables critical protections.
Q: Do all scanners read i3 battery DTCs accurately?
A: Only BMW ISTA, Autel MaxiSys Elite, or Foxwell NT510 show full sub-code details. Generic OBD2 tools miss critical data.
Q: Will a used OEM pack throw fewer codes?
A: Often the opposite—used packs have hidden degradation that triggers intermittent Red/Yellow codes within months.
Q: Does CNS provide DTC support after purchase?
A: Yes—our engineers offer free remote diagnostics for any code on CNS-installed packs.
Q: Are these codes the same for REx and BEV models?
A: Yes—the high-voltage battery system is identical across all i3 variants (2014–2022).
Don’t Interpret Codes in the Dark
With the right knowledge, every DTC becomes a roadmap—not a riddle.
Install a Pack Designed to Stay Code-Free From Day One
Stop treating symptoms. Start preventing failures.
Order your CNS BMW i3 battery—engineered to eliminate the most common and dangerous DTCs—or request our printable 2026 DTC Quick Reference Card:
👉 https://cnsbattery.com/ev-battery-home/ev-battery-contact/