AutoCodes – Generic – B3904

B3904 Code - Rear Compartment Occupant Sensor Signal Invalid

Quick Answer

Rear compartment occupant sensor is connected to battery voltage and ground at all times. Common causes include Faulty Rear Compartment Occupant Sensor, Rear Compartment Occupant Sensor harness is open or shorted, Rear Compartment Occupant Sensor circuit poor electrical connection. Severity is High - diagnose this code as soon as possible. Driving is not recommended if the warning light is flashing, the engine runs poorly, or safety-related symptoms are present.

What it means: B3904 indicates Rear Compartment Occupant Sensor Signal Invalid.
Common causes: Common causes include Faulty Rear Compartment Occupant Sensor, Rear Compartment Occupant Sensor harness is open or shorted, Rear Compartment Occupant Sensor circuit poor electrical connection.
Severity: High - diagnose this code as soon as possible.
Can I drive?: Driving is not recommended if the warning light is flashing, the engine runs poorly, or safety-related symptoms are present.

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Code Popularity 687

How popular is this code, among other codes? Low

Repair Importance Level

Urgent (Diagnose now) (3 / 3)

Repair Difficulty Level

Advanced (Pro) (3 / 3)

What is this?

Table of contents Created and updated by William Orellana on Mar 25, 2020

Description
Possible Causes
How To Fix
Cost to Diagnose
FAQs
Comments
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Code B3904 Description

Rear compartment occupant sensor is connected to battery voltage and ground at all times. When a combination of a difference in temperature compared to ambient and movement is detected by rear compartment occupant sensor and ignition is off, Body Control Module (BCM) will command horn to chirp and command deck lid to release within 3 minutes of a detect signal from rear compartment occupant sensor. If ignition is on when BCM receives detect signal, disable switch on rear compartment occupant sensor will flash.
Rear compartment occupant sensor can be disabled by pressing disable switch on rear compartment occupant sensor and holding for 2 seconds. BCM will command trunk ajar telltale on for one minute with each ignition on signal while rear compartment occupant sensor is disabled. Once deck lid is opened, rear compartment occupant sensor will reset to on and resume normal operation. Rear compartment occupant sensor can also be enabled inside of rear compartment with ignition on or off by pressing disable switch. Deck lid ajar switch signal circuit is used for rear compartment occupant sensor class 2 data.
The Diagnostic Trouble Code (DTC) sets when BCM receives an invalid status signal from rear compartment occupant sensor.

OBDII Code B3904 - Rear Compartment Occupant Sensor Signal Invalid - AutoCodes.com — B3904 Code - Rear Compartment Occupant Sensor Signal Invalid

What does this mean?

What are the Possible Causes of the DTC B3904?

Faulty Rear Compartment Occupant Sensor
Rear Compartment Occupant Sensor harness is open or shorted
Rear Compartment Occupant Sensor circuit poor electrical connection
Faulty Body Control Module (BCM)

How to Fix the DTC B3904?

Review the 'Possible Causes' above and visually examine the corresponding wiring harness and connectors. Check for damaged components and inspect connector pins for signs of being broken, bent, pushed out, or corroded.

What is the Cost to Diagnose the Code?

Labor: 1.0

To diagnose the B3904 code, it typically requires 1.0 hour of labor. Rates vary by location, vehicle, and shop. Many shops charge between $80–$150 per hour; dealerships and metro areas may be higher, independents may be lower.

Need more information about the B3904 code?

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Need Help with the Code?

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Information for Specific Makes

B3904 Buick — Rear Compartment Occupant Sensor Signal Invalid
B3904 Cadillac — Rear Compartment Occupant Sensor Signal Invalid
B3904 Chevrolet — Rear Compartment Occupant Sensor Signal Invalid
B3904 GMC — Rear Compartment Occupant Sensor Signal Invalid

About the author: William Orellana is a Master Mechanic and Mechanical Engineer with over 30 years of experience diagnosing and repairing vehicles. Learn more about his background.

Frequently Asked Questions about B3904 Code

1. What are the common symptoms of OBDII code B3904 Rear Compartment Occupant Sensor Signal Invalid?

The common symptoms include the airbag warning light illuminating on the dashboard and a potential disabling of the rear occupant airbag system.

2. What are the possible causes of OBDII code B3904 Rear Compartment Occupant Sensor Signal Invalid?

Possible causes include a faulty rear compartment occupant sensor, damaged wiring or connectors in the rear compartment sensor circuit, or a malfunction in the airbag control module.

3. How can I diagnose the issue related to OBDII code B3904?

Diagnosing the issue requires using a diagnostic scanner to retrieve the trouble codes, followed by inspecting the rear compartment occupant sensor, checking the wiring and connectors for any damage, and testing the airbag control module for proper functionality.

4. What are the potential risks of ignoring OBDII code B3904 Rear Compartment Occupant Sensor Signal Invalid?

Ignoring this code can lead to the rear occupant airbag system not deploying in the event of a crash, potentially compromising the safety of rear passengers in the vehicle.

5. Can I repair OBDII code B3904 Rear Compartment Occupant Sensor Signal Invalid on my own?

While some basic troubleshooting steps can be performed at home, it is recommended to seek professional help for diagnosing and repairing issues related to airbag systems to ensure safety and proper functioning.

6. How much does it typically cost to repair OBDII code B3904 Rear Compartment Occupant Sensor Signal Invalid?

The cost of repair can vary depending on the exact cause of the issue, but it may involve replacing the rear compartment occupant sensor, repairing damaged wiring, or reprogramming the airbag control module. Costs can range from a few hundred to over a thousand dollars.

7. Is it safe to drive with OBDII code B3904 Rear Compartment Occupant Sensor Signal Invalid?

It is not advisable to drive with this code present, as it indicates a potential issue with the rear occupant airbag system, which could compromise passenger safety in the event of a collision.

8. How long does it take to repair OBDII code B3904 Rear Compartment Occupant Sensor Signal Invalid?

The repair time can vary depending on the specific repair needed and the availability of parts. In general, it may take a few hours to a full day to diagnose and repair the issue.

9. Can a low car battery trigger OBDII code B3904 Rear Compartment Occupant Sensor Signal Invalid?

A low car battery can sometimes cause electrical issues that may trigger this code, but it is essential to thoroughly diagnose the root cause to ensure the problem is properly addressed.

10. Are there any recalls related to OBDII code B3904 Rear Compartment Occupant Sensor Signal Invalid?

Manufacturers occasionally issue recalls related to airbag systems, so it is advisable to check with your vehicle's manufacturer or the National Highway Traffic Safety Administration (NHTSA) to see if there are any relevant recalls for your specific vehicle and issue.