What this code means
B0021 may mean the SRS unit is detecting an open circuit or increased resistance in the left side curtain airbag inflator circuit.
What the vehicle may do
- The SRS warning light may be on.
- The supplemental restraint system may limit airbag operation while the fault is active.
Possible fault areas
- A possible issue with the left side curtain airbag inflator connection or inflator circuit.
- A possible open or high-resistance condition in the floor wire harness.
- A possible poor connection at the SRS unit connector.
- A possible SRS unit or SRS unit software-related issue.
Diagnostic path
Opening and safety setup
On this Civic, B0021 points us toward the left side curtain airbag inflator circuit. In plain terms, the SRS unit may be seeing an open circuit or higher-than-expected resistance on that side curtain airbag path. The SRS light may be on, and the system may limit airbag operation while the fault is active. Broadly, the possible fault areas can include the inflator connection, the floor wire harness, the SRS unit connection, or the SRS unit itself. Before getting into the test path, ask whether the vehicle has been in a collision. If it has, make sure the required SRS components were replaced with new correct-number parts and installed properly. Also follow normal SRS handling precautions and battery disconnect and reconnect practices. This is not a circuit to probe casually.
Confirm the code is repeatable
Start by confirming the fault. Clear the DTCs with the HDS. Turn the ignition switch to ON (II), then wait for 10 seconds. Now check DTCs again with the HDS. If B0021-13 is indicated again, the failure is duplicated and you move into the circuit isolation checks. If it does not come back, treat it as an intermittent failure and the system is OK at this time. If other codes are present, check what they mean first instead of chasing this one in isolation.
Isolate the left side curtain airbag inflator
Next, isolate the inflator from the circuit. Turn the ignition switch to LOCK (0). Disconnect the negative cable from the battery, then wait at least 3 minutes. Remove the left side C-pillar trim, and disconnect the left side curtain airbag inflator 2P connector on the floor wire harness. Connect the terminals of SRS simulator lead L to the SRS inflator simulator using the 2 Ω connectors, then connect simulator lead L to that left side curtain airbag inflator 2P connector on the floor wire harness. Reconnect the negative battery cable. Clear the DTCs with the HDS, turn the ignition switch to ON (II), wait for 10 seconds, and check DTCs again. If B0021-13 is no longer indicated, the fault is open or increased resistance in the left side curtain airbag inflator; replace the left side curtain airbag, then clear the DTC. If B0021-13 is still indicated, keep going into the open-wire check.
Check the LCA positive and negative lines for an open
For the open-wire check, turn the ignition switch back to LOCK (0). Disconnect the negative cable from the battery and wait at least 3 minutes. Disconnect SRS unit connector B, the 39P connector. Then disconnect the SRS inflator simulator from SRS simulator lead L, but leave simulator lead L connected to the left side curtain airbag inflator 2P connector on the floor wire harness. Measure resistance at the SRS simulator lead L black terminal, checking the LCA positive and LCA negative circuits with the ignition at LOCK (0), the SRS unit connector disconnected, and the floor-harness side connector connected through simulator lead L. The decision point is 1.0 Ω or less. If the measurement is 1.0 Ω or less, the LCA positive and LCA negative wires are OK. At that point, inspect the connection between SRS unit connector B and the SRS unit. Before replacing the SRS unit, check the SRS unit software version with the HDS; if it is not the latest version, update it and retest. If the connection is OK and the software check does not resolve it, replace the SRS unit. If the measurement is not 1.0 Ω or less, the fault is open or increased resistance in the floor wire harness; replace the floor wire harness, then clear the DTC.
Verify the repair
Keep verification separate from the testing. After the supported repair, clear the DTCs, turn the ignition switch to ON (II), wait for 10 seconds, and confirm with the HDS that B0021-13 stays gone. The takeaway is simple: prove the code repeats, use the simulator to separate the inflator from the harness and control side, then use the resistance check to decide whether the LCA wiring path is intact. For more diagnostic training, visit stepdiagnostics.com.
Final check
This diagnosis can separate an inflator-side problem from a harness or SRS unit connection problem by confirming the code, substituting the simulator, and then checking the LCA circuit resistance.
For more guided automotive diagnostics, visit STEP Diagnostics.
