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B1904 OBD-II Trouble Code: Air Bag Crash Sensor #2 Feed/Return Circuit Failure

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B1904 code definition

The B1904 trouble code notes a problem with the airbag crash sensor #2 circuit.

What the B1904 code means

The B1904 code is a generic OBD-II trouble code that detects a circuit malfunction in the #2 airbag crash sensor. The #2 crash sensor is one of multiple crash sensors, which detect when the vehicle encounters severe impact, and trigger the airbags to deploy. Different vehicles have different numbers of crash sensors, located in different positions, so the placement of the #2 crash sensor will vary depending on the make and model. If the airbag control module, body control module, or powertrain control module (PCM) notice an abnormal voltage reading from the #2 airbag crash sensor circuit, then the B1904 trouble code may be detected.

What causes the B1904 code?

The B1904 trouble code is commonly caused by two things. Damaged electrical components, such as shorted wires, corroded connectors, blown fuses, or an open circuit are the most common culprit. A defective #2 airbag crash sensor is also fairly common. While very rare, it is also possible for a faulty airbag control module, PCM, or body control module to be the cause of the BB1904 code.

What are the symptoms of the B1904 code?

When the B1904 code is detected, the check engine soon Warning Light and airbag Warning Light will most likely illuminate on the vehicle’s gauge cluster. In most cases, the airbag system will not work, so the vehicle will likely be without airbags should an accident occur.

How does a mechanic diagnose the B1904 code?

The B1904 code will be diagnosed with the help of a standard OBD-II trouble code scanner. A trained mechanic will view the freeze frame data from the scanner to assess the B1904 code and search for additional trouble codes. If multiple codes are noted, then the mechanic will need to address them in the order that the scanner displays them in. The trouble codes will then be reset, the vehicle restarted, and the B1904 code checked for. If the code is no longer present, then an intermittent error or an inaccurate detection are the likely culprits.

If the B1904 code remains detected following the reset, then a visual inspection will be performed on the airbag and crash sensor electrical system. Any damaged components will need to be replaced. The mechanic will then test the #2 airbag crash sensor circuit, and then inspect the crash sensor itself.

Any time the mechanic replaces a component, the trouble codes should again be reset, the vehicle restarted, and the B1904 code checked for. This will help the technician know as soon as the issue is resolved.

Common mistakes when diagnosing the B1904 code

The most commonly made error when diagnosing the B1904 code comes from failing to follow the OBD-II diagnosis protocol. The diagnosis protocol should be followed step by step at all times, as it helps ensure that diagnoses are thorough, and repairs are accurate.

Often airbag crash sensors are unnecessarily replaced because damaged electrical components were overlooked.

How serious is the B1904 code?

A vehicle with the B1904 code will still be drivable, and will not have any drivability concerns. However, the airbag system will likely not work, which means that there will be no airbags deployed should the vehicle crash. Airbags are an extremely important safety feature that can save the lives of drivers and passengers, so a vehicle with this code should not be driven until it has been inspected.

What repairs can fix the B1904 code?

Repairs for the B1904 trouble code are:

  • Replacement of wires, connectors, and fuses
  • Replacement of the #2 airbag crash sensor circuit
  • Replacement of the #2 airbag crash sensor
  • In rare cases, replacement of the airbag control module, body control module, or PCM

Additional comments for consideration regarding the B1904 code

The B1904 code is a manufacturer specific trouble code, meaning it has a different definition depending on the automaker. While this code signals a #2 airbag crash sensor circuit malfunction in many cars, it can have an entirely different meaning for other cars. As is always the case, technicians should take into account the vehicle they are working on when diagnosing the B1904 code.

Need help with a B1904 code?

YourMechanic offers certified mobile mechanics who will come to your home or office to diagnose and repair your vehicle. Get a quote and book an appointment online or speak to a service advisor at 1-800-701-6230.

The statements expressed above are only for informational purposes and should be independently verified. Please see our terms of service for more details
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