Bxxxx · Body
B2784
Engine Immobilizer / Anti-Theft Authentication Failure
CriticalBody ECU
What it means
The vehicle's immobilizer system did not receive a valid key authentication. Engine will crank but not start. Common Toyota / Lexus code.
Common causes
Listed in rough order of frequency. Verify with vehicle-specific service info before parts replacement.
- 01Faulty transponder key (worn or damaged chip)
- 02Failed transponder antenna / immobilizer coil around the ignition
- 03Disconnected or failed immobilizer ECU
- 04Aftermarket remote-start system interfering with immobilizer signal
- 05Battery in smart key (push-button-start vehicles)
- 06Recently replaced ECM that was not properly synced to the immobilizer
Affected systems
ECUs and subsystems that report or are affected by this code.
Immobilizer ECUEngine Control Module (ECM)Body Control Module (BCM)
Get vehicle-specific diagnosis for B2784
The corpus has 14,000+ indexed forum threads. Ask with your vehicle (year + make + model) and we'll synthesize an answer cited to the original posts. First 5 questions free.
Ask the forums about B2784Critical-severity code
This code typically prevents the vehicle from starting, running normally, or operating safely. Diagnose carefully before clearing — clearing without fixing the root cause often masks deeper problems.