BMW i8 (2014–2020) Won't Start & No Power
No power (12V system de-energised)
Non-start condition
HV system would not start despite request
The following fault code(s) may be present
21F105 - High voltage battery cell supervision circuit 3 - temperature measurement of the cell block
22281C - High-voltage battery cell supervision circuit 3 - temperature measurement of the cell block
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Parts Repaired or Replaced
- HV battery safety-box (safety fuse assembly)
- EME (Electric Motor Electronics) control unit (replacement unit)
Affected Vehicles
BMW i8
- BMW i8 2014
- BMW i8 2015
- BMW i8 2016
- BMW i8 2017
- BMW i8 2018
- BMW i8 2019
- BMW i8 2020
Common Failure Symptoms
- Complete no-power condition (12V system de-energised)
- Vehicle will not start (HV system not initialising)
- HV contactors commanded but not closing
- HV safety device/fuse status fault codes stored
BMW i8 Diagnostic Report
Initially we investigated a non-start condition and confirmed the vehicle had no electrical power, with the 12V system fully de-energised.
Subsequently, the HV system retained fault codes indicating the battery positive terminal contactor was stuck open or the safety fuse had released, and that the HV system would not start despite request. Live data showed the contactor control was active but the contactors were not closing, suggesting an internal safety fuse disconnection.
BMW i8 Repairs
We ran an ISTA session to log the repair and confirm the pyro/safety device status via guided diagnostics. Results indicated the HV safety fuse was disconnected internally, so the HV battery pack was removed for internal checks and HV bus short testing. A replacement safety-box was installed and the pack refitted for further validation.
- Ran ISTA session to confirm internal safety fuse disconnection
- Removed HV battery pack; carried out internal checks and HV bus short checks
- Installed replacement safety-box and resealed battery lids
- Removed, inspected, and replaced EME (tested for isolation and bus resistance)
- Reset systems, verified fault codes cleared, and completed extensive road test under load
Resolution
After replacing the HV battery safety-box and fitting a tested replacement EME, the vehicle was reassembled, reset, and checked for fault codes. An extensive road test confirmed stable operation with no warning messages or errors under load conditions.
Following the repair:
- Vehicle started and operated normally
- No recurring HV fault codes recorded
- Road test confirmed no errors under load
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