What this code means
B0021 may indicate an open circuit or increased resistance in the left side curtain airbag first inflator circuit.
What the vehicle may do
- The SRS warning indicator may be on.
- The SRS system may store the code after detecting a circuit fault.
- Airbag system operation may be affected while the fault is present.
Possible fault areas
- The left side curtain airbag inflator circuit may be involved.
- Connector fit or terminal contact at the inflator or SRS unit may be involved.
- The left or right side wiring harness may have an open circuit.
- The SRS unit software or SRS unit may be involved.
Diagnostic path
Opening and safety setup
On this Accord, B0021 is an SRS code for the left side curtain airbag first inflator circuit. In plain terms, the SRS unit may be seeing an open circuit or increased resistance on that inflator path. The driver may notice an SRS warning indicator, and the possible fault area can include the curtain airbag inflator connection, the left or right side wiring, the SRS unit connection, SRS unit software, or the SRS unit itself. Before touching the circuit, find out whether the vehicle has been in a collision. If it has, make sure the required SRS components were replaced with new, correct-part-number components and that they were installed properly. Also slow down and handle this like an airbag job: follow the SRS safety precautions and the proper 12 volt battery disconnect and reconnect process before unplugging anything.
Confirm the fault first
Start by confirming the fault. Clear the codes with the HDS, turn the vehicle to ON mode, wait for 10 seconds, and check codes again with the HDS. If B0021-13 comes right back, the fault is duplicated and you move into circuit isolation. If it does not return, treat it as an intermittent at this moment. The system is OK right now, so pause and work it as an intermittent concern. If other codes are present, check what they mean first instead of chasing this code in isolation.
Isolate the curtain airbag inflator side
Next, check whether the left side curtain airbag inflator is the part of the circuit causing the open or increased resistance. Turn the vehicle to OFF, disconnect the negative cable from the 12 volt battery, and wait at least 3 minutes. Remove the headliner, then disconnect the left side curtain airbag inflator 2P connector at the left side wire harness. On the harness-side connector, jumper terminal No. 1 to terminal No. 2. Reconnect the negative battery cable, clear the codes with the HDS, turn the vehicle to ON mode, wait for 10 seconds, and check for B0021-13 again. If the code is no longer indicated with that jumper in place, the diagnostic path calls the left side curtain airbag inflator faulty: replace the left side curtain airbag, then clear the code. 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 harness check, turn the vehicle back to OFF, disconnect the negative cable from the 12 volt battery again, and wait at least 3 minutes. Remove the jumper from the left side curtain airbag inflator 2P connector. Then disconnect SRS unit connector B, the 39-pin connector. At SRS unit connector B, jumper terminal No. 9 to terminal No. 10. With the vehicle OFF, the curtain airbag inflator 2P connector disconnected, SRS unit connector B disconnected, and terminals No. 9 and No. 10 jumped, measure resistance between the two terminals at the left side curtain airbag inflator 2P connector on the left side wire harness: No. 1 to No. 2. The decision point is 1.0 Ω or less.
Use the resistance result to choose the next move
If the resistance is 1.0 Ω or less, the LCA positive and LCA negative wires are considered OK on this check. At that point, the remaining path is the SRS unit side: look closely for a poor connection at 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 the SRS unit software and retest. If the connection is OK and the software path has been handled, replace the SRS unit. If the code still does not clear after that, replace the left or right side wire harness. If the resistance is not 1.0 Ω or less, the path points to an open in the left or right side wire harness. Replace the faulty harness, then clear the code.
Verification and takeaway
Keep the verification separate from the circuit testing. After the directed repair or software step, clear the code and confirm B0021-13 stays gone under the same basic check: ON mode, wait 10 seconds, then recheck with the HDS. The takeaway is simple: prove the code first, simulate the inflator at the harness connector, then use the 1.0 Ω or less harness check to separate an inflator concern from wiring, connector, software, or SRS unit concerns. For more diagnostic training, visit stepdiagnostics.com.
Final check
This diagnosis is about isolating whether the code follows the inflator side, the harness side, or the SRS unit connection and software path.
For more guided automotive diagnostics, visit STEP Diagnostics.
