B0120low severityBody
Theft Deterrent System Malfunction
on 2021 Mazda Mazda3 2.5L I4 · FWD
What this code means
A general fault in the vehicle theft deterrent (anti-theft) system. The system may trigger false alarms or not arm properly.
Symptoms
- Security light flashing rapidly
- False alarm activation
- System will not arm
- Horn chirp confirmation missing when locking
Common causes
- Faulty hood/door/trunk switch
- BCM anti-theft module failure
- Wiring issue
- Aftermarket alarm interfering
- Battery was disconnected
Diagnostic steps
- Scan BCM for theft deterrent codes
- Check all door, hood, and trunk switches
- Inspect for aftermarket alarm system
- Verify BCM programming
- Test alarm arm/disarm cycle
Repair notes
Repair faulty switches or wiring. Remove conflicting aftermarket alarm if present.
Other Body codes on this vehicle
B0001
Driver Frontal Stage 1 Deployment Control
B0002
Driver Frontal Stage 2 Deployment Control
B0003
Passenger Frontal Stage 1 Deployment Control
B0004
Passenger Frontal Stage 2 Deployment Control
B0005
Driver Knee Airbag Deployment Circuit
B0006
Passenger Knee Airbag Deployment Circuit
B0007
Rear Left Side Airbag Deployment Circuit
B0008
Rear Right Side Airbag Deployment Circuit
B0009
Rear Center Airbag Deployment Circuit
B0010
Driver Side Airbag Deployment