What this code means
B0091 may indicate the SRS unit is not receiving the expected signal from the left side impact sensor circuit.
What the vehicle may do
- The SRS warning indicator may stay on.
- Some restraint system functions may be limited while the fault is active.
- The fault may be intermittent if the code does not return after clearing.
Possible fault areas
- Possible connector or terminal fit issues in the left side impact sensor circuit.
- Possible wiring faults between the SRS unit and the left side impact sensor area.
- Possible left side impact sensor concern.
- Possible SRS unit or SRS unit software concern after circuit checks pass.
Diagnostic path
Opening context
On this Accord, B0091 may mean the SRS unit is not seeing the expected signal from the left side impact sensor circuit. The SRS warning may be on, and depending on the fault, restraint system operation may be limited while the fault is active. In broad terms, this can come from a connector issue, the left side impact sensor area, wiring between the sensor and the SRS unit, or in some cases the SRS unit side of the system. We’re going to keep the diagnosis focused on the no-signal path for B0091 and work it in order.
Safety and pre-checks
Before touching the SRS circuit, find out whether the vehicle was in a collision. If it was, verify the required SRS parts are new, correct by part number, and installed properly. Also handle the SRS system and the 12 volt battery disconnect and reconnect steps carefully. Start with the basic system checks and follow a structured diagnostic approach, especially if other codes are present. Check what they mean first instead of chasing the wrong branch.
Confirm the B0091 fault
First, clear the DTCs with the HDS. Turn the vehicle to the ON mode, wait for 10 seconds, and check for DTCs again. If the B0091 no-signal fault is present, continue with the left side impact sensor circuit checks. If no DTC returns, treat it as an intermittent condition and the system is OK at this time. If the scan result points to a different left side impact sensor path, do not force it through this branch.
Check the main connectors first
Turn the vehicle to the OFF (LOCK) mode. Check the SRS unit connector B, 39P, the left side impact sensor first 4P connector, and the left side impact sensor second 2P connector. If the connectors are OK, move on to sensor substitution. If a loose connection is found, reset the connection that caused the problem, clear the DTCs with the HDS, turn the vehicle to the ON mode, wait for 10 seconds, and check again. If B0091-87 does not return, the system is OK at this time. If it does return, continue testing.
Substitute the first left side impact sensor
With the vehicle in OFF (LOCK) mode, substitute a known-good left side impact sensor first. Reconnect the negative cable to the 12 volt battery, clear the DTCs with the HDS, turn the vehicle to the ON mode, wait for 10 seconds, and check for DTCs. If B0091-87 is gone with the known-good sensor, replace the left side impact sensor first and clear the DTC. If B0091-87 is still indicated, leave the sensor question behind and start checking the wiring.
Prepare for short checks
Turn the vehicle to the OFF (LOCK) mode. Disconnect the negative cable from the 12 volt battery and wait at least 3 minutes. Then disconnect SRS unit connector B, 39P, the left side impact sensor first 4P connector, and the left side impact sensor second 2P connector. From here, the goal is to prove the signal lines are not shorted together, not shorted to ground, and not shorted to power.
Check for shorts between paired lines
First check the LS1 pair. Measure resistance between the left side impact sensor first 4P connector No. 2 and No. 3. You want an open circuit, or at least 1 MΩ. If it passes, the LS1＋ and LS1－ wires are not shorted together, and you continue. If it fails, you have a short to another wire in the harness section between the SRS unit and the first left side impact sensor, so do not continue down the pass side of the path. Next check the LS2 pair. Measure resistance between the left side impact sensor first 4P connector No. 1 and No. 4. Again, you want an open circuit, or at least 1 MΩ. If it passes, the LS2＋ and LS2－ wires are not shorted together. If it does not pass, the fault is a short to another wire in the left side wire harness between the first and second left side impact sensors; replace the left side wire harness, then clear the DTC.
Check for shorts to ground
Now check those same circuits to body ground. For the LS1 lines, measure from the left side impact sensor first 4P connector No. 3 to body ground, and then from No. 2 to body ground. Each check should show an open circuit, or at least 1 MΩ. If not, there is a short to ground in the left or right side wire harness between the SRS unit and the first left side impact sensor; replace the faulty harness, then clear the DTC. Then check the LS2 lines. Measure from the left side impact sensor first 4P connector No. 1 to body ground, and then from No. 4 to body ground. Each one should also be open, or at least 1 MΩ. If not, there is a short to ground in the left side wire harness between the first and second left side impact sensors; replace the left side wire harness, then clear the DTC.
Check for shorts to power
Reconnect the negative cable to the 12 volt battery and turn the vehicle to the ON mode. Keep the SRS unit connector B, the first sensor 4P connector, and the second sensor 2P connector disconnected for this test. For the LS1 lines, measure voltage from the left side impact sensor first 4P connector No. 3 to body ground, and then from No. 2 to body ground. The result should be 0.2 V or less. If it is higher, there is a short to power in the left or right side wire harness between the SRS unit and the first left side impact sensor; replace the faulty harness, then clear the DTC. For the LS2 lines, measure from the left side impact sensor first 4P connector No. 1 to body ground, and then from No. 4 to body ground. That result should also be 0.2 V or less. If it is higher, there is a short to power in the left side wiring between the first and second left side impact sensors; replace the left wire harness, then clear the DTC.
Check for an open in the LS1 circuit
Next, turn the vehicle back to OFF (LOCK) mode. At SRS unit connector B, 39P, connect terminal No. 38 to terminal No. 39 with a jumper wire. Then measure resistance between the left side impact sensor first 4P connector No. 2 and No. 3. You want 1.0 Ω or less. If the reading is not 1.0 Ω or less, there is an open in the left or right side wire harness between the SRS unit and the first left side impact sensor; replace the faulty harness, then clear the DTC. If the reading is 1.0 Ω or less, the LS1＋ and LS1－ wires are OK. Disconnect and reconnect the connector, then check the DTC again. If the DTC is still present, check the SRS unit software version with the HDS before replacing the SRS unit. If the software is not the latest, update it and retest.
Companion left-side sensor path context
There is a companion path in this left-side impact sensor diagnosis. Keep it as context for this episode, not as the main B0091 path. If the scan result or connector decision sends you there, it starts with the same connector check: vehicle OFF (LOCK), then inspect SRS unit connector B, 39P, the left side impact sensor first 4P connector, and the left side impact sensor second 2P connector. If a loose connection is found on that path, reset the connection that caused the problem, clear the DTCs with the HDS, turn the vehicle to the ON mode, wait for 10 seconds, and check again. If that companion no-signal code is still indicated, continue with its open-wire check. For that open-wire check, turn the vehicle to OFF (LOCK), disconnect the negative cable from the 12 volt battery, and wait at least 3 minutes. Disconnect the first sensor 4P connector and second sensor 2P connector. Then jump the first sensor 4P connector No. 1 to No. 4, and measure resistance at the second sensor 2P connector between No. 1 and No. 2. The pass point is 1.0 Ω or less. If it passes, those LS2 lines are OK and the next action is to replace the second left side impact sensor, then clear the DTC. If it does not pass, there is an open in the left side wire harness between the first and second left side impact sensors; replace the left side wire harness, then clear the DTC.
Verify the repair
Keep verification separate from the circuit testing. After the repair action for the failed B0091 branch is completed, clear the DTCs with the HDS, turn the vehicle to the ON mode, wait for 10 seconds, and check that B0091 stays gone. The takeaway is simple: confirm the code first, clean up connector issues before deeper testing, prove shorts before opens, and do not condemn the SRS unit until the wiring and software checks support it. For more diagnostic training, visit stepdiagnostics.com.
Final check
B0091 is often approached as an electrical signal-path diagnosis: confirm the code, check connections, substitute only where the path calls for it, and then prove the wiring before judging the control
For more guided automotive diagnostics, visit STEP Diagnostics.
