B0021 indicates a fault in the left curtain airbag deployment circuit (Stage 1). This code is set by the Supplemental Restraint System (SRS) module when it detects an abnormal resistance or electrical issue in the deployment loop for the left side curtain airbag.
The airbag inflator module itself may have internal short, open, or high resistance. This is a common failure due to age or manufacturing defect.
The harness between the SRS module and the left curtain airbag can suffer from chafing, broken wires, or corrosion at the connector pins, especially under the headliner or near door hinges.
Loose or improperly seated connectors, or bent pins, can cause intermittent or permanent high resistance in the deployment circuit.
Less common, but the SRS module itself may have an internal failure affecting the deployment circuit. This should only be considered after thoroughly checking wiring and the airbag.
The definition is universal; the diagnostics are engine-specific. Open your vehicle for the codes documented for it:
No. B0021 indicates a safety system fault. The airbag may not deploy in a crash, or could deploy unexpectedly. The code will likely return if the underlying issue is not fixed.
No. This code indicates an electrical fault in the deployment circuit, not that the airbag has fired. A deployed airbag would typically show a different code or no code at all (module may be destroyed).
No. The PCM (powertrain control module) is not involved in the airbag system. B0021 is an SRS code, and the control module involved is the SRS module. Replacing the PCM will not fix this code.
The vehicle may be driven, but the left curtain airbag will not function in a side-impact or rollover crash. Some jurisdictions may consider this a safety violation. Have the system repaired as soon as possible.