B1421

B1421 means the SRS/airbag control module sees the side crash sensor module circuit reporting open — the side-impact sensor mounted in the B-pillar or front door is either electrically disconnected, has a broken internal squib bridge, or has shifted out of its mounting tolerance enough that the module can no longer verify it. On most modern platforms each side crash sensor is a 2-wire device with a measured squib resistance of 1.5-3.0 ohms between the pins when intact; an open circuit reads infinite ohms. Diagnostic order: (1) DO NOT probe airbag circuits with a standard DMM under power — back-probing under battery voltage can fire a live squib. Disconnect the battery, wait at least 10 minutes for the SRS capacitor to discharge per FSM spec, then ohm the sensor circuit. (2) Inspect the sensor mounting in the B-pillar or door — sensors are torque-spec critical, typically 8-12 Nm, and a loose sensor or one mounted with a missing isolator washer will throw an open even if the sensor itself is fine. (3) Inspect the wiring harness through the door jamb / B-pillar grommet — repeated door slams fatigue the wires inside the rubber boot. (4) Replace the sensor with an OEM part (aftermarket SRS components are a no-go for liability and crash performance). Misdiagnosis caveat: side-crash sensors are mounted in the B-pillar or door — if the airbag light came on after a quarter-panel hit, door slam from a parking-lot bump, or an aftermarket speaker install, the sensor often just shifted out of mounting tolerance or has a chafed pigtail; reseat and retorque before quoting a $200-$500 sensor plus dealer programming.

Vehicle-specific patterns: 2002-2016 Honda Accord/Civic/CR-V/Odyssey across multiple model years carry NHTSA recalls for Takata passenger-side and driver-side inflators that throw a constellation of SRS codes including B1421 alongside open/short codes on the front impact sensors — always run the VIN through the NHTSA recall database before quoting any airbag work, because the airbag itself may be free at the dealer. 2008-2010 Ford F-250/F-350/F-450 SuperDuty commonly throws B1421 alongside a no-start of the Occupant Classification Sensor (OCS) zero-set procedure — the OCS mat under the passenger seat needs to be rezeroed via IDS after any seat R&R, and a missed rezero throws SRS codes including B1421. 2005-2010 Chevy Cobalt, Pontiac G5, and HHR carry the well-known ignition-switch / clock-spring recall that drives B1421 alongside steering-wheel airbag codes — recall coverage is open at GM dealers for affected VINs. 2003-2010 Mercedes-Benz E-class W211 throws B1421 from rear seat-belt buckle pretensioner harness chafe at the seat track. Estimated repair: $0 (NHTSA recall coverage on Takata or GM ignition switch) to $850 (OEM side crash sensor plus airbag module reset).