What this code means
B0021 can indicate an open circuit or increased resistance in the left side curtain airbag first inflator.
An open circuit is a break in an electrical path. Increased resistance means the path has more resistance than the procedure allows. For B0021, the path under investigation belongs to the left side curtain airbag first inflator. The diagnosis therefore needs to separate a problem associated with that component from a problem elsewhere in its circuit.
The left side curtain airbag first inflator, its connecting paths and the SRS unit are different possible fault areas. Naming the left side curtain airbag first inflator in the code does not make those possibilities interchangeable. The steps below narrow the fault area by changing a defined test condition and then checking the individual paths.
What the vehicle may do
- The SRS unit can store this code for high detection-line voltage.
Possible fault areas
- The left side curtain airbag may have a problem.
- The LCA+ or LCA- circuit may have an open wire.
- The SRS unit may have a problem.
The procedure moves through confirmation, an indirect component check and electrical-path checks. In the indirect check, the specified jumper connection is used exactly as described. The result determines whether the component or other related parts remain involved. The later measurements examine LCA+ and LCA- through the wiring between the curtain airbag connector and SRS unit.
Read the result criteria alongside the action that produces them. During the component check, a normal circuit-test result can point to a problem with the named component. During a wire check, an abnormal reading instead points to the checked path. These conclusions belong to different stages of the diagnosis.
Diagnostic path
Understand the stored condition
B0021 means Open or Increased Resistance in the Left Side Curtain Airbag First Inflator. The SRS unit stores this code when voltage on its internal detection line is higher than a specified value.
Safety and collision history
Before you start, follow the SRS precautions and the 12 volt battery terminal disconnection and reconnection requirements. Find out whether the vehicle was in a collision. If it was, make sure all required components were replaced with new components of the correct part number. Make sure those components were also installed properly.
Initial confirmation
Turn the vehicle to the ON mode. Wait for 10 seconds.
Check the curtain airbag
Before you disconnect any SRS related connector, follow the 12 volt battery terminal disconnection procedure. Wait for at least 3 minutes. Disconnect the left side curtain airbag inflator connector (2P). Install a jumper wire between terminals No. 1 and No. 2 of that connector. Repeat the confirmation check: turn the vehicle to the ON mode, then wait for 10 seconds. DTC B0021-13 should not be indicated for a normal result. If the result is normal, there is a problem with the left side curtain airbag.
If the code is still indicated, there is a problem with other related components.
For the component test, a normal result means the stated DTC B0021-13 is not indicated after the specified test connection and confirmation. The procedure associates that normal test result with a problem in the left side curtain airbag first inflator. An abnormal result follows the branch for other related components.
Here, the word normal refers to the response of the circuit test. It does not describe a direct resistance measurement of the left side curtain airbag first inflator. The wire checks that follow use their own resistance criteria and therefore answer a different question. This distinction is the key to reading the component and circuit branches correctly.
Prepare for the wiring checks
Check wires LCA+ and LCA- for an open wire next. Before you disconnect any SRS related connector, follow the 12 volt battery terminal disconnection procedure. Wait for at least 3 minutes. Disconnect SRS unit connector B (57P). Make sure the left side curtain airbag inflator connector (2P) is also disconnected.
Check the LCA+ wire
Check wires LCA+ and LCA- for an open circuit, starting with LCA+. Keep IG Mode OFF for this check. To check resistance, place one probe on SRS unit connector B (57P) terminal 25.
- Other probe: left side curtain airbag inflator connector (2P) terminal 2.
The reading should be less than 1 ohm. If the reading is outside that normal range, there is a problem in the circuit.
The LCA+ and LCA- paths are separated by their terminal pairs. LCA+ pairs SRS unit connector B terminal 25 with curtain-airbag connector terminal 2. LCA- pairs SRS terminal 26 with airbag terminal 1. These pairs identify the individual paths being measured.
The result of the earlier jumper check is whether the specified DTC remains indicated under the confirmation condition. The result here is resistance through a named wire path, with less than 1 ohm as the normal criterion. Keeping those two kinds of result separate explains how the procedure distinguishes the curtain airbag branch from a circuit branch.
Check the LCA- wire
Keep IG Mode OFF for this check as well. To check resistance, place one probe on SRS unit connector B (57P) terminal 26.
- Other probe: left side curtain airbag inflator connector (2P) terminal 1.
The reading should be less than 1 ohm. If the reading is outside that normal range, there is a problem in the circuit.
When the other checks are normal
If all the other checks give normal results, the remaining cause is the SRS unit.
Final check
The jumper check and the two wire readings can point toward the curtain airbag, its circuit, or the SRS unit.
Taken together, the results form a chain. The indirect component check establishes which branch to follow; the individual electrical checks identify an abnormal path; and the final SRS-unit conclusion depends on the earlier inspection items being normal. Keep the actual result beside the component or wire that was being checked. That makes the final decision traceable to the procedure, rather than to the wording of the stored code alone.
