U0028 – Vehicle area network (VAN) data bus A

By Stephen Darby (Contact Me)
Last Updated 2016-07-01
ASE Master Tech

Table of Contents

1. What Does Code U0028 Mean?
2. What are the common causes of code U0028?
3. What are the symptoms of code U0028?
4. How do you troubleshoot code U0028?
5. Codes Related to U0028
6. Get Help with U0028

What Does Code U0028 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.

What are the common causes of code U0028?

Common causes may include:

• Defective power relays
• Faulty or blown fuses or fusible links
• Cut, burnt, or damaged wiring harnesses or connectors
• A defective PCM or other controller
• PCM or controller programing errors

What are the symptoms of code U0028?

Depending upon what area of the VAN in which a malfunction has occurred, the vehicle may exhibit the following symptoms:

• Engine drivability concerns
• Erratic transmission shift patterns
• (Exterior or interior) lighting malfunctions or inoperative lighting
• Illumination of the service engine soon lamp, traction control lamp, or antilock brake lamp
• Diminished climate control system performance
• Inoperative or erratic speedometer or odometer operation
• Power door lock or power window malfunctions

How do you troubleshoot code U0028?

To correctly diagnose this code, you will need some type of diagnostic scanner (or code reader), a digital volt/ohmmeter (DVOM), and a manufacturer’s service manual. Due to the extensive number of wiring circuits that can be involved in diagnosing this code, a specialty tool (such as an AutoHex) may be useful. In some instances, an oscilloscope may be required as well.

You might start your diagnosis of a U0028 code with a visual inspection of system wiring and connectors. Don’t forget an inspection of the battery, battery cables, and battery terminal ends, as a low voltage condition may cause this type of code to be stored. Also make sure that the alternator is charging sufficiently. Clean, repair, or replace corroded battery cables and battery cable ends as necessary. Damaged, burnt, or corroded wiring, connectors, and/or components should also be repaired or replaced, as required. System fuses, circuit breakers, and power supply relays (and connectors) should be tested at this time, as well. Replace defective parts as necessary.

NOTE: System fuses may appear to be operational with the vehicle in the “key on engine off” (KOEO) position and fail when the circuit is placed under a load. Test system fuses with the vehicle in the “key on engine running” (KOER) position and as many functions as possible in operation.

You can continue by retrieving and making a notation of all stored trouble codes and available freeze-frame data before clearing the codes.

To continue your diagnosis, you must determine which section of the VAN or individual controller is specified as “data bus A.” The manufacturer’s service manual should be helpful in determining this information but a secondary service, such as All Data or Mitchell On Demand, will yield much more useful information. Because of the complex diversity of this particular code, symptoms should factor heavily into your diagnosis as well. If there are problems with the instrument panel or gauges (or if you have additional codes that imply such a problem), suspect a malfunction in the VAN between the instrument panel control module and the PCM. An antilock brake system code, accompanied by a U0028, would indicate that there is a problem with the VAN that is related to the antilock braking module and so forth. Additionally, other stored codes can normally lead you in the right direction when this type of code is stored. If this code is stored alone, suspect a defective control module or a programming error. Diagnose and repair other codes before attempting to diagnose this code.

Use the scanner (if available) to determine if the PCM is recognizing an input signal from the controller in question. If there is an input signal from the controller in question, and this code is stored, there could be a PCM malfunction or a programming error.

If the PCM fails to recognize any inputs from the controller or VAN in question, consult the manufacturer’s service manual for system wiring diagrams, connecter face views, and desired circuit values. Disconnect all related controllers from the VAN and perform a circuit resistance and continuity test on each individual circuit. This process can be physically demanding and very time consuming. Using a DVOM for circuit resistance testing may damage system controllers. Repair or replace all shorted or open circuits and recheck the system.

Codes Related to U0028

Here are some related possible codes:

• PCM speed sensor codes
• Transmission control codes
• Instrument panel codes
• Antilock brake system codes
• Body control module codes

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