B1921: Air Bag Diagnostic Monitor Ground Circuit Open
What does B1921 mean?
B1921 is a diagnostic trouble code indicating "Air Bag Diagnostic Monitor Ground Circuit Open". This means there is an electrical break in the restraint system wiring — a connector may be unplugged, a wire broken, or a component has failed internally. The ECU detected no signal where it expected one.
Symptoms you may notice
- Check engine light (MIL) illuminated on dashboard
- Intermittent warning lights
- Erratic gauge readings
Estimated repair cost
Typical range in Australia (parts + labour)
Common causes
- Damaged or corroded wiring harness
- Loose or disconnected electrical connector
- Failed restraint system sensor or actuator
- Blown fuse in the related circuit
- Poor ground connection
What should you do?
This code sits in the occupant restraint system. The airbags or pretensioners may not fire in a crash, and on some faults they can fire when they should not. The car will usually drive and feel completely normal, which is exactly why this one gets ignored. Have it diagnosed before you carry passengers, and do not clear the code to make the light go away.
Get a diagnosis for YOUR car
Code B1921 can mean different things depending on your vehicle. TorqueBot gives you a diagnosis specific to your make, model, and year.