B0004 Code Explained: Driver Knee Bolster Deployment Control (Subfault)

If your OBD-II scanner reports B0004, your vehicle has detected a problem within the driver knee bolster airbag deployment circuit. While this code won’t affect how your engine runs, it can disable part of your vehicle’s Supplemental Restraint System (SRS), reducing protection during a collision.

Modern vehicles use multiple airbags—including knee airbags—to reduce lower-body injuries and help position occupants correctly during frontal impacts. The B0004 code indicates the Airbag Control Module (ACM) has detected an electrical fault in the driver’s knee bolster deployment circuit.

Here’s everything you need to know about the B0004 code, including symptoms, causes, diagnosis, repair costs, and how to fix it.

Quick Facts

Item	Information
OBD-II Code	B0004
Description	Driver Knee Bolster Deployment Control (Subfault)
Category	Body (B)
System	Supplemental Restraint System (SRS)
Severity	★★★★★ (High)
Safe to Drive?	Mechanically yes, safety-wise no
Repair Difficulty	Moderate
Typical Repair Cost	$150–$1,400

What Does B0004 Mean?

The B0004 trouble code indicates the Airbag Control Module has detected an abnormal electrical condition in the driver knee bolster airbag deployment circuit.

Many modern vehicles include a knee airbag beneath the steering column. During a frontal collision, it deploys upward to:

Reduce knee and lower leg injuries
Help keep the driver properly positioned
Prevent excessive forward movement
Improve seatbelt effectiveness

If the ACM detects excessive resistance, low resistance, an open circuit, or a short in the deployment circuit, it stores B0004 and illuminates the Airbag Warning Light.

Depending on the manufacturer, the affected airbag—or the entire SRS system—may be disabled until repairs are completed.

Symptoms of B0004

Drivers commonly notice:

Airbag warning light illuminated
SRS warning message
Stored B0004 code
Additional body or SRS diagnostic codes
Driver information display warning
No noticeable drivability issues

The vehicle usually drives normally because the problem affects a safety system rather than the engine or transmission.

Common Causes

1. Damaged Knee Airbag Wiring

Since the knee airbag is mounted beneath the steering column, wiring can become damaged from:

steering column repairs
pedal work
aftermarket remote start installations
accidental impacts

2. Loose or Corroded Connectors

Poor electrical connections create increased resistance that the ACM detects as a deployment circuit fault.

3. Faulty Knee Airbag Module

Internal failure of the inflator assembly may prevent the deployment circuit from passing self-tests.

Although possible, this is less common than wiring problems.

4. Airbag Control Module Failure

Water damage, collision damage, or electrical spikes can cause the ACM to incorrectly detect deployment faults.

5. Previous Collision Repairs

Improper repairs often leave:

damaged harnesses
improperly seated connectors
incorrect replacement airbags
aftermarket resistors

These issues frequently trigger B0004.

6. Rodent Damage

Vehicles stored for long periods sometimes develop damaged wiring from rodents chewing harnesses beneath the dashboard.

How Serious Is B0004?

Extremely serious.

Although your vehicle remains mechanically safe to operate, the Supplemental Restraint System may not provide full occupant protection during a collision.

A disabled knee airbag can increase the risk of lower-body injuries and may also affect the timing of other restraint systems.

Can You Drive With B0004?

Technically, yes.

Should you?

No.

An illuminated SRS warning light indicates the airbag system cannot guarantee proper deployment during an accident.

Prompt repairs are strongly recommended.

How to Diagnose B0004

Step 1: Read SRS Codes

Use a professional scan tool capable of accessing:

Airbag Module
Body Control Module
Supplemental Restraint System

Basic OBD-II scanners typically cannot read B0004.

Step 2: Inspect Under-Dash Wiring

Carefully inspect wiring beneath the steering column for:

loose connectors
damaged insulation
corrosion
signs of previous repairs

Step 3: Check Companion Codes

Related codes often include:

B0001
B0002
B0010
B0012
B0020

Multiple deployment circuit faults may indicate a shared wiring issue or ACM failure.

Step 4: Measure Circuit Resistance

Using factory service procedures, verify resistance throughout the deployment circuit.

Never use a standard test light on SRS wiring.

Step 5: Inspect the Knee Airbag Assembly

If wiring tests normally, inspect the knee airbag module itself for internal faults or collision damage.

Common Repair Solutions

Repair	Estimated Cost
Repair wiring harness	$150–450
Replace connector	$100–300
Replace knee airbag module	$600–1,200
Replace ACM	$900–2,000
Clear codes and verify repair	$75–150

Common Diagnostic Mistakes

Many technicians assume the airbag itself has failed.

In reality, most B0004 faults originate from:

damaged wiring
connector corrosion
loose terminals
poor previous repairs

Replacing expensive airbag components without testing the electrical circuit often wastes both time and money.

Vehicles That Commonly Experience B0004

This code can appear on many vehicles equipped with knee airbags, including:

Chevrolet Silverado
GMC Sierra
Ford Explorer
Ford Edge
Lincoln Navigator
Cadillac XT5
Toyota Camry
Toyota Highlander
Lexus RX
Honda Accord
Honda Pilot
Acura MDX
Nissan Maxima
Nissan Murano
Hyundai Palisade
Kia Telluride

Manufacturer-specific descriptions may vary slightly.

How to Prevent B0004

While not every failure can be prevented, these practices help reduce the risk:

Avoid damaging wiring beneath the dashboard
Disconnect the battery before steering column repairs
Never splice SRS wiring
Repair collision damage using OEM components
Address intermittent airbag warning lights immediately
Protect interior wiring from water intrusion

Frequently Asked Questions

What causes B0004?

The most common causes include damaged wiring, poor electrical connections, a faulty knee airbag module, or an Airbag Control Module malfunction.

Will B0004 clear itself?

No.

The code remains stored until the underlying electrical problem is repaired and the SRS system passes its self-test.

Can a weak battery cause B0004?

Low voltage may trigger temporary airbag warnings, but persistent B0004 faults almost always indicate an actual deployment circuit issue.

Is the knee airbag important?

Yes.

Knee airbags help reduce leg injuries and improve occupant positioning during frontal collisions, allowing other restraint systems to work more effectively.

Can I replace the knee airbag myself?

Because the Supplemental Restraint System contains explosive components, repairs should follow factory procedures. Improper handling can result in accidental deployment and serious injury.

Will the airbag warning light turn off automatically?

After the repair is completed and codes are cleared, the warning light should remain off if the system passes all diagnostic checks.

Final Thoughts

The B0004 code may not affect how your vehicle drives, but it directly impacts one of your most important safety features. The driver’s knee airbag plays a vital role in reducing injuries during a frontal collision, making prompt diagnosis and repair essential.

Fortunately, many B0004 issues are caused by damaged wiring or poor electrical connections rather than a failed airbag module. Proper diagnosis can often save hundreds of dollars by identifying the true source of the fault before replacing expensive components.

Expand your SRS diagnostics with these guides: