B0091 indicates a fault in the circuit for the left side restraints sensor 1, which is part of the supplemental restraint (airbag) system. The SRS control module has detected an abnormal signal, open, short, or no response from that sensor. This usually turns on the airbag warning light and may disable the affected side-impact protection.
The sensor itself can fail internally, producing an out-of-range, open, or shorted signal that the SRS module cannot trust.
The harness running through the B-pillar, door jamb, or rocker panel can chafe, break, or lose connection from flexing and moisture.
Poor contact at the sensor plug or inline connector can increase resistance and cause intermittent or persistent communication errors.
Incorrectly routed wiring, pinch damage, or added electrical devices on the sensor circuit can alter signals and trigger the code.
If the sensor and wiring test good, a rare internal failure in the SRS control module may misread a normal signal, but this is much less common.
The vehicle will usually drive normally, but the left side airbags may be disabled, so you should have it diagnosed and repaired promptly.
It may turn the light off temporarily, but the code and warning will return if the underlying sensor or wiring fault is still present.
No. This is an SRS sensor circuit code and does not involve the engine PCM. Replacing the PCM is almost never the correct fix; the problem is usually the sensor, its connector, or the wiring.
Very low system voltage can sometimes cause temporary SRS communication errors, but a persistent B0091 almost always points to the left side sensor circuit itself.