B1821 means the airbag sensor assembly has measured an open circuit on one of the side airbag firing loops. Toyota writes it as B1821/55. On most models that loop belongs to the front seat side airbag on the right, and on others it belongs to the driver seat, so the service data for that year and model decides which seat comes apart.
The side squib circuit runs from the airbag sensor assembly under the console, through the floor wire, and out to the side airbag built into the seat back. Toyota's description of the circuit is short: it consists of the airbag sensor assembly and the front seat airbag assembly. The module measures that loop continuously.
Toyota gives an exact set condition. The airbag sensor assembly has to receive an open circuit signal in the side squib circuit for two seconds before B1821/55 stores. The shorted versions of the same loop are faster to set, with a short to ground or a short to B+ storing after half a second. The trouble areas Toyota lists for all four codes are identical: the floor wire, the front seat airbag assembly, and the airbag sensor assembly.
Because the airbag lives in the seat, the wiring has to cross a joint that moves. The floor wire feeds a connector under a seat that slides and tilts every day, and that connector and the harness around it are where opens usually happen.
Circuit, with one common exception. Day to day, B1821 is a wiring or connector report on a vehicle that was never hit in the side. The exception is a car that did take a hit there. A fired squib no longer completes a loop, so a deployed seat airbag reads as an open, and Toyota's own trouble area list includes the airbag assembly for that reason.
After a side collision the module carries its own record of the event, and clearing that is separate work from replacing the bag and repairing the harness.
Only in the crash case, and only once the hardware is right. If nothing deployed, restoring the module's memory does nothing for an open connector under a seat. Reconnect or repair the circuit and the light goes out on the next key cycle.
If the side airbag did fire, the sequence is a correct replacement seat airbag, a repaired or replaced floor wire, then an airbag module reset so your original sensor assembly comes back clear. Genuine OEM seat side airbags are searchable by vehicle, and belts that locked in the same impact are handled through seat belt repair.
B1821/55 is a Toyota code, and Lexus uses the same numbering on shared platforms. Which seat it names is the part to confirm before any work starts. Service data for the Tacoma, Venza, and C-HR groups B1820/55 through B1823/55 under Side Squib RH Circuit, RAV4 data titles that block driver side, side squib circuit, and a 2004 Camry manual words B1820/55 as a short in the side squib D seat side circuit.
Disconnect the battery, wait out Toyota's discharge time, then get under the seat the manual points to and look at the connector first. A yellow SRS connector that was never pushed home is the cheapest outcome, and it happens on any vehicle that has had a seat out. If the connector is clean and latched, move to the floor wire and work back toward the module.
Read the neighboring codes before ordering anything. B1820 is a short across the same pair, B1822 is a short to ground, B1823 is a short to B+, and B1826 covers the opposite side. B1801 is the driver frontal version of an open firing loop, and B1620 reports the side airbag sensor rather than the firing circuit.
Related codes: B1820, B1822, B1826, B1801
Keep reading:
Code scope: Toyota. The same number can mean something different on another manufacturer, so match the code to your make before you act on it.
A reset clears stored crash data. A replacement part fixes a hardware fault. We have read these codes since the nineties and will tell you which one you are looking at.
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