What does it mean?
When a diagnostic trouble code U0404 is presented, it generally indicates that the powertrain control module (PCM), transmission control module (TCM), or body control module (BCM) have detected an input signal from a particular area of the vehicle area network (VAN) that is not within a pre-programmed limit.
Vehicle area network data buses are a line signaling protocol that is responsible for the transmission of data between various engine, transmission, and body controllers, throughout the vehicle. You may be familiar with controller area networks (CAN), simply because they are more common than VANs.
The VAN differs from the CAN only slightly; both networks utilize a line signaling protocol that signals ones and zeroes but the VAN uses a phase encoder (also known as Manchester) method that sets it apart. It encodes blocks of four-bits as three-non-return-to-zero encoded bits, followed by a Manchester encoded bit. Both the CAN and VAN are forms of highly-efficient serial data communication that are represented by what auto technicians would think of as waveform patterns.
Virtually every function of the OBD-II equipped vehicle is accomplished using electronic devices. These devices are controlled with individual controllers or modules. The PCM normally controls the engine and transmission, but some vehicles are also equipped with a TCM, which controls transmission functions. The BCM handles multiple chassis related function, such as lighting, climate control, and safety equipment operation.
There are also instrument panel control modules, climate control modules, supplemental inflatable restraint control modules, electronic brake control modules (EBCM) for antilock brakes, and dozens more control modules of differing varieties. Communication between the various control modules is absolutely vital to reduced exhaust emissions and fuel efficiency. Although these individual control modules operate independently of one another, they must also interact by sharing system data.
For instance; the TCM depends upon the PCM for engine RPM levels and the EBCM needs the TCM for comparison of wheel speed sensor data – and so on and so forth. The VAN is the way that these different control modules communicate. Since it would not be practical to create a unique wiring harness for each mass-production vehicle, automakers adopted the practice of producing an identical and specific wiring harness for each model.
Despite option and feature differences, each harness is designed to accommodate whatever control modules may be required. This pre-fabricated harness contains the VAN. For example: An identical VAN is used in vehicles that are equipped with automatic transmissions as those that are equipped with manual transmissions. The only difference is that the automatic transmission equipped cars may have a TCM plugged into the VAN and the vehicle that is equipped with a manual transmission will not.
This durable, uniform design keeps production costs down and provides a more reliable VAN that is much easier to diagnose if a problem occurs.