OBD-II trouble code
B004A: Third Row Right Frontal Stage 2 Deployment Control (Subfault)
A deployment loop assigned to the third row right position has failed its electrical check. Third row restraint circuits are the longest and worst-routed wiring in the vehicle — they cross the rear wheel well area and pass under seats that fold — which is why corrosion and crushing dominate here in a way they never do at the front.
Quick facts
- System
- Body
- Category
- Airbag / SRS Restraints
- Severity
- High severity
- Drivable
- Usually safe to drive short-term
- Repair cost range
- $100 – $1,200
- DIY difficulty
- Shop recommended
What does B004A mean?
This code belongs to the small group of restraint faults that can only appear on a vehicle with third row provisions — a large SUV, a van, or a three-row crossover. That already tells you something useful, and it tells you something else worth confirming: the physical device the manufacturer assigned to this loop varies. On some vehicles it is a seat-mounted device for the rearmost occupants, on others a curtain extension covering the third row, and on others again a knee or seat cushion device. The SAE wording names a control loop and a seating position, not a specific bag, so the wiring diagram for the exact year, model and body style is the only thing that tells you what is actually on the end of the circuit. Ordering a part before that is confirmed is a good way to buy the wrong one.
What is reliably true regardless of which device is fitted is the route the wiring takes, and that is the most useful fact on this page. The restraints module sits near the centre of the vehicle, typically under the console or beneath the front seats. A third row circuit therefore has to travel most of the length of the car to reach its device. It runs along the sill or under the floor, past the rear wheel well, and up into the rearmost seating area. Every part of that journey is worse than the equivalent journey to a front airbag. The rear sill sections get walked on by everyone climbing into the back. The underfloor run beside the rear wheel is the wettest and most heavily salted part of the underbody, and grommets there fail first. On vehicles where the harness passes under a folding seat, it is subjected to repeated crushing every time the seat is stowed and raised. Corrosion and mechanical damage account for far more third row restraint faults than failed inflators do, which is the opposite of the usual assumption.
The folding seat point deserves emphasis because it produces a specific and recognisable pattern. A device mounted in a seat that folds has a connector that moves, and a harness section that flexes, every time the seat is operated. Circuits like that fail intermittently long before they fail permanently, and the fault correlates with seat use rather than with driving. If the warning light appears after a weekend of carrying cargo with the seats folded and clears itself later, that history is worth more than any measurement. Similarly, a rear seat that has been removed and refitted for cargo — common on vans and large SUVs — routinely leaves a restraint connector unlatched.
As with all restraint faults, the vehicle drives normally and the risk is confined to what happens in a crash. The module will normally refuse to fire a circuit it cannot verify, so the outcome is a device that does not deploy rather than one that deploys unexpectedly. These are pyrotechnic circuits and the system must be disabled by the manufacturer's procedure before anything is unplugged.
Common causes
- Corrosion in the underfloor harness run beside the rear wheel well, which is the wettest and most heavily salted part of the route
- Harness crushed under a folding third row seat mechanism from repeated stowing and raising
- Connector left unlatched after a rear seat was removed or refitted for cargo
- Failed grommet where the harness passes through the floor pan, allowing water into the loom
- Rear sill harness section damaged by occupants climbing in and out over it
- Connector at the device corroded from water ingress through a tailgate or rear quarter leak
- Damage during aftermarket work in the rear of the vehicle, such as flooring, towing electrics or audio installation
- Failed squib in the device assigned to this loop, which is genuine but less common than the wiring causes
- Restraints module configured for a build with third row provisions the vehicle does not have
Symptoms
- Airbag or SRS warning light on
- B004A stored with a symptom byte describing the electrical condition
- Fault appearing or clearing in step with the third row seats being folded and raised
- Code arriving after the rear seats were removed for cargo and put back
- No effect at all on engine, steering or braking
- Other rear restraint or rear body faults stored alongside it where the shared harness run is damaged
- Corrosion visible on rear underbody connectors during inspection
- Warning light appearing seasonally on vehicles driven on salted roads
Diagnostic steps
- 1.Confirm from the wiring diagram what device the manufacturer actually assigned to this loop on this exact model and body style. Seat-mounted, curtain extension and cushion devices are all possible and they are not interchangeable.
- 2.Ask how the third row seats have been used recently. A fault that appears and disappears in step with folding or removing the seats is a flexing harness or an unlatched connector, and that history saves hours.
- 3.Verify the restraints module is configured for a vehicle with third row provisions. A module coded for the wrong build monitors circuits that were never installed.
- 4.Disable the restraint system by the manufacturer's procedure and observe the specified waiting period before opening any connector.
- 5.Inspect the connector at the device first, checking that it is fully latched. Seats that come out for cargo are a routine source of connectors that never went back together.
- 6.Follow the harness under the folding seat mechanism, looking for crushing, chafe and pinched sections at the pivot points.
- 7.Inspect the underfloor run beside the rear wheel well and its grommets closely. This is the corrosion-prone section and it is where third row restraint circuits most often fail.
- 8.Check the rear sill harness where occupants step over it, which is exposed to both mechanical damage and water carried in on footwear.
- 9.Measure the deployment loop against specification only after the connector, the seat mechanism and the underfloor run have been inspected, since wiring outnumbers component failure heavily on this circuit.
Repair cost
$100 – $1,200
Third row restraint work carries more labour than the equivalent front repair because access is poor and seats often have to come out. A connector that is simply unlatched may cost nothing beyond the $100 to $200 diagnostic. A harness repair under a folding seat or in the sill is $200 to $500 once the seat and trim are removed. Repairing a corroded underfloor section can reach $400 to $700 because the run is long and awkward. Replacing the device itself typically lands between $500 and $1,200 fitted depending on what it turns out to be, which is exactly why identifying the device from the wiring diagram before quoting matters here more than on a front airbag code.
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.