OBD-II trouble code
B0042: Third Row Left Frontal Stage 2 Deployment Control (Subfault)
A firing-loop fault at the third-row left position. This loop is the longest restraints circuit in the vehicle and it terminates under a load floor, which is why water and cargo damage account for so many of these.
Quick facts
- System
- Body
- Category
- Airbag / SRS Restraints
- Severity
- High severity
- Drivable
- Usually safe to drive short-term
- Repair cost range
- $130 – $1,000
- DIY difficulty
- Shop recommended
What does B0042 mean?
B0042 is logged by the restraints module against the second firing stage of a frontal airbag at the left-hand third-row seat. The vehicle drives exactly as it did before; what is compromised is a device that only matters during a collision.
Everything distinctive about this code comes from where the third row lives. It is the last row in the vehicle, so its firing loop is the longest restraints run on the car — the wiring leaves the module in the centre of the vehicle and travels the full length of the floor, under sill trim, past every seat-belt anchor and seat rail, to a connector that terminates in the lowest and most rearward part of the body. Length alone means more opportunities for damage, but the location is the bigger factor.
The third row is also the row that gets treated as luggage space. It is folded flat, tumbled forward, sometimes removed entirely, and then walked on and loaded with cargo. Whatever sits under that floor is under a surface that people stand on, drop things onto and slide heavy objects across. Its connector is the one most likely to be crushed by a load rather than to fail electrically on its own.
Water is the other half of the story. The rearmost floor is where water goes: a perished tailgate or liftgate seal, a blocked drain, a leaking rear light gasket or a wet load left in the back all end up in the lowest part of the body, which is exactly where this connector sits. Restraints connectors are gold-plated and sealed precisely because the system cannot tolerate resistance drift, but a connector standing in water for a season will corrode anyway. On a vehicle used for sport, camping, trade work or anything that puts wet items in the back, corrosion at the under-floor connector should be the first thing inspected, not the last.
A third and less obvious cause belongs to this position specifically. Third-row seating is very often a later addition or a secondhand part — seats transplanted from a higher trim, sourced from a breaker to convert a five-seat vehicle, or replaced after damage. A seat assembly that looks identical can carry a different inflator, a different loop resistance or no frontal squib at all, and the module will report the mismatch as a fault in the loop. Establishing where the seat came from is a legitimate diagnostic question here.
Finally, this is the region of the vehicle where aftermarket work happens: rear entertainment, amplifiers and subwoofers, cargo lighting, trailer wiring and tracking devices are all installed under and around this floor, frequently by fitters who have no reason to know a restraints loop passes beneath them. A self-tapping screw through a floor pan or a loom cut and rejoined for convenience will produce this code weeks later. Ask what has been fitted before assuming age.
Common causes
- Corrosion at the under-floor connector after water ingress from a tailgate seal, blocked drain or wet cargo
- Connector or harness crushed by cargo loaded onto the folded third-row floor
- Damage from the seat being folded, tumbled or removed and refitted without routing the loom correctly
- Screws, brackets or fasteners from aftermarket rear installations driven through or against the loop
- Secondhand or transplanted third-row seat carrying a different inflator or no frontal squib at all
- Chafe along the longest restraints run in the vehicle, at sill trim clips and seat-belt anchor points
- Open or out-of-specification stage two squib in the third-row left inflator
- Restraints module configured for a seating layout the vehicle does not have
- Failed drive circuit in the restraints control module, confirmed last
Symptoms
- Airbag warning lamp on and remaining on
- B0042 stored with a symptom byte describing an open or out-of-range loop
- Fault that first appeared after the rear of the vehicle was carrying a wet or heavy load
- Fault dating from an aftermarket audio, lighting, trailer or tracker installation
- Damp carpet or standing water in the rearmost footwell or load area
- Fault that changes state when the third-row seat is folded or raised
- Fault present since a third-row seat was fitted, replaced or sourced secondhand
- Vehicle starting, idling and driving completely normally
Diagnostic steps
- 1.Lift the load floor covering and check the rearmost floor for damp, staining and standing water before any electrical testing, since water ingress is the leading cause at this position.
- 2.Inspect the under-floor restraints connector for green corrosion, moisture inside the housing and crush damage from cargo.
- 3.Ask what has been installed in the rear of the vehicle — audio, lighting, trailer wiring, trackers — and inspect for fasteners driven into or against the loom.
- 4.Establish whether the third-row seat is original to the vehicle or was added or replaced, and confirm the inflator matches what the module expects.
- 5.Record the full symptom byte and note which other third-row codes are present, since a single-loop fault and a whole-row fault lead in different directions.
- 6.Power down and discharge the restraints system to the manufacturer's procedure before disconnecting anything.
- 7.Trace the loop along its full length under the sill trim, checking at every clip, anchor bolt and seat rail where it is pinched or rubbed.
- 8.Fold, tumble and latch the third-row seat through its full travel while monitoring for an intermittent.
- 9.Measure the loop resistance with approved restraints test equipment rather than a general-purpose meter.
- 10.Repair the cause of any water ingress as well as the connector, then clear the codes and confirm the lamp stays out over several journeys.
Repair cost
$130 – $1,000
Cleaning and re-terminating a corroded under-floor connector is $150 to $420, and fixing the leak that caused it — a tailgate seal, a blocked drain, a light gasket — adds $60 to $300 but is the part that stops it recurring. Repairing the loop where it has been crushed or drilled through runs $180 to $450, most of it in access and tracing. A third-row inflator or seat-mounted assembly is $300 to $800 in parts on the applications that carry one. Restraints module replacement with programming reaches $1,000. Diagnosis is $110 to $220.
Estimate your repair
Run the numbers for your vehicle
Open the Repair Cost Estimator with airbag control module (sdm) replacement / reset preselected. Adjust labor rate and vehicle category to fit your situation.
DIY vs shop
Leave this one to a qualified shop. It typically involves emissions-critical components, refrigerant handling, or other work that requires manufacturer-grade tooling, training, or certification. DIY attempts often produce a more expensive problem than the original code.