What this code means
B0090 may mean the SRS unit is not receiving a usable signal from the left front impact sensor circuit.
What the vehicle may do
- The SRS warning light may be on.
- The airbag system may limit normal operation while the fault is active.
- The fault may appear intermittently if the connection or wiring concern is not present all the time.
Possible fault areas
- Possible connector concerns at the SRS unit, left front impact sensor, or the connector between them.
- Possible wiring faults in the left front impact sensor signal lines.
- Possible left front impact sensor concern.
- Possible SRS unit concern after the circuit, sensor path, and software status are checked.
Diagnostic path
Opening context
On this 2010 to 2015 Honda Civic with the 1.8 gas engine, B0090 points us toward the left front impact sensor circuit in the SRS system. In plain terms, the SRS unit may not be seeing a usable signal from that sensor. The vehicle may turn on the SRS warning light, and the airbag system may limit normal operation while the fault is active. The fault area can include the impact sensor, connector issues, the left front sensor wiring between the engine compartment harness and dash harness, or the SRS unit side of the circuit. Before getting into electrical testing, find out whether the vehicle has been in a collision. If it has, make sure the required SRS components were replaced with new parts of the correct part number and properly installed. Also handle this like any SRS job: follow the SRS safety precautions, general troubleshooting approach, and the correct battery disconnect and reconnect procedure.
Verify the fault first
Start by verifying the problem. Clear the codes with the HDS. Turn the ignition switch to ON (II), wait for 10 seconds, and check codes again with the HDS. If B0090-87 comes right back, the failure is duplicated and you move into connector checks. If B0090-87 does not come back, treat it as an intermittent condition and do not force a repair that is not currently failing. If other codes are present, check what they mean first.
Check the connectors before circuit testing
Next, turn the ignition switch to LOCK (0). Check SRS unit connector A, the 39P connector, the left front impact sensor 2P connector on the engine compartment wire harness, and the connector between the SRS unit and the left front impact sensor. If those connectors are clean, secure, and OK, continue to the circuit checks. If a connector problem is found, repair the poor connection, clear the codes with the HDS, turn the ignition switch to ON (II), wait for 10 seconds, and check again. If B0090-87 is gone, the system is OK at this time. If the code returns, turn the ignition back to LOCK (0) and continue with the circuit tests.
Check for a short between the sensor lines
For the first wiring check, disconnect the negative battery cable and wait at least 3 minutes before unplugging SRS connectors. With ignition at LOCK (0), disconnect SRS unit connector A, the 39P connector, and disconnect the left front impact sensor 2P connector. Measure resistance between SRS unit connector A terminal No. 28 and terminal No. 29. You are checking the LFS＋ and LFS－ lines against each other. An open circuit, or at least 1 MΩ, means those two wires are not shorted together, so continue to the next check. If the reading is not an open circuit or at least 1 MΩ, the problem is a short to another wire in the engine compartment wire harness or dashboard wire harness. The supported repair path is to replace the faulty harness, then pause and verify the earlier diagnostic path instead of guessing past the incomplete reset direction.
Check for a short to ground
With the SRS unit connector A and the left front impact sensor connector still disconnected, measure each sensor line to body ground. For LFS＋, measure from SRS unit connector A terminal No. 28 to body ground. For LFS－, measure from terminal No. 29 to body ground. An open circuit, or at least 1 MΩ, means those lines are not shorted to ground, so continue. If the reading is not an open circuit or at least 1 MΩ, the fault is a short to ground in the engine compartment wire harness or dashboard wire harness. Replace the faulty harness, then clear the DTC.
Check for a short to power
Now reconnect the negative battery cable and turn the ignition switch to ON (II). Leave SRS unit connector A and the left front impact sensor 2P connector disconnected. Measure voltage from SRS unit connector A terminal No. 28 to body ground for LFS＋, and from terminal No. 29 to body ground for LFS－. The expected result is 0.2 V or less. If both lines are 0.2 V or less, they are not shorted to power and you can move to the open-wire check. If the reading is not 0.2 V or less, the fault is a short to power in the engine compartment wire harness or dashboard wire harness. Replace the faulty harness, then clear the DTC.
Check for an open in the sensor circuit
Turn the ignition switch back to LOCK (0). Connect the black terminal of SRS simulator lead L to the SRS inflator simulator jumper connector, then connect SRS simulator lead L to the left front impact sensor 2P connector on the engine compartment wire harness. With SRS unit connector A still disconnected, measure resistance between SRS unit connector A terminal No. 28 and terminal No. 29. This checks continuity through the LFS＋ and LFS－ lines using the simulator lead. The expected result is 1.0 Ω or less. If it is not 1.0 Ω or less, there is an open in the engine compartment wire harness or dashboard wire harness. Replace the faulty harness, then clear the DTC. The connector-terminal callouts for this simulator setup need to be followed carefully; if the connection point is not certain, stop and recheck the setup rather than guessing.
Sensor and SRS unit decision
If the open-wire check is 1.0 Ω or less, the LFS＋ and LFS－ wiring checks OK. At that point, replace the left front impact sensor, clear the DTC, and retest. If the code is still present after that retest, check the SRS unit software version with the HDS before replacing the SRS unit. If the software is not the latest, update the SRS unit software and retest first. The final SRS unit replacement wording is partially cut off, so do not skip the software check or guess at the ending of the path.
Takeaway and CTA
Bottom line: for B0090-87, prove the code is active, inspect the connectors first, then work the left front impact sensor circuit in order: short between lines, short to ground, short to power, and open circuit. Keep verification separate from testing, and do not condemn the SRS unit until the wiring, sensor path, retest, and software check support it. For more diagnostic training, visit stepdiagnostics.com.
Final check
B0090 is often handled as a sensor circuit diagnosis: verify the code, check the connectors, test the wiring in order, then make the sensor or SRS unit decision only after the earlier checks supportها
For more guided automotive diagnostics, visit STEP Diagnostics.
