Damaged, corroded or loose connector at the left curtain airbag or its in-line connector; Wiring fault in the left curtain airbag circuit, such as chafing, a break or a short, often in the A, B or C pillar or roof lining area; Faulty left curtain airbag module (squib resistance out of range); Disturbed wiring or connectors after trim, headlining, seat or interior work; Water ingress or corrosion in the roof or pillar area affecting the circuit; Fault within the SRS control module itself.

B0021

Left Curtain Deployment Control 1.

Generic
Body
Generic code
High severity

How this code is built

B: SystemBody
0: Defined bySAE, the same on every car
0: AreaBody area 0
21: FaultThe specific fault in that area

What it means

B0021 is a generic body code meaning a fault has been detected in the left curtain airbag deployment control circuit 1. The restraint control module (SRS module) checks the squib circuit for resistance, shorts to voltage or ground, and open circuits, and it sets the code when a reading falls outside the expected range. The exact circuit layout and fault wording depend on the vehicle, so check the manufacturer's data for the model.

Symptoms

Airbag or SRS warning light on the dashboard
Light may stay on constantly or come on intermittently
Left curtain airbag may not deploy in a crash while the fault is present
Other airbag fault codes stored alongside it
Failed MOT inspection due to the SRS warning light

Possible causes

Damaged, corroded or loose connector at the left curtain airbag or its in-line connectorlikely
Wiring fault in the left curtain airbag circuit, such as chafing, a break or a short, often in the A, B or C pillar or roof lining arealikely
Faulty left curtain airbag module (squib resistance out of range)possible
Disturbed wiring or connectors after trim, headlining, seat or interior workpossible
Water ingress or corrosion in the roof or pillar area affecting the circuitpossible
Fault within the SRS control module itselfrare

How to diagnose

Read all stored and pending codes and note any other SRS or related codes, then record the freeze frame or status data if available
Ask about recent accident damage, interior trim or headlining work, or seat removal that could have disturbed the circuit
Visually inspect the left curtain airbag connector and any in-line connectors for damage, corrosion, bent pins or a loose fit
Check that any connector locks or shorting clips are correctly seated and not damaged
Inspect the visible wiring along the left pillars and roof edge for chafing, pinching or water damage
Follow the manufacturer's safety procedure: disconnect the battery and wait the required time before testing any airbag circuit, and never use a standard multimeter or test light on a squib circuit
Use the manufacturer's recommended method to check circuit resistance and live SRS data to see whether the fault lies in the wiring, the airbag or the module
After repair, clear the codes and confirm they do not return

How to fix

Repair or replace damaged or corroded connectors and terminals using the correct parts
Repair or replace damaged wiring in the left curtain airbag circuit to the manufacturer's specification
Replace the left curtain airbag module if it is confirmed faulty, following the correct procedure
Refit and secure any wiring or connectors disturbed during previous work
Replace or reprogram the SRS control module only if testing confirms it is at fault and the manufacturer specifies it

Can I keep driving?

High severity

The car may be driven short distances to reach a workshop, but the left curtain airbag may not deploy in a crash, so repair it promptly. Avoid carrying passengers on the left side where possible until it is fixed.

Guide written by AI from the code's definition. Confirm with a diagnosis before replacing parts.