U0003, U0004, U0005, U0006, U0007, and U0008: High Speed CAN Communication Bus

These are common codes that indicate a problem with the high speed CAN bus wires or the CAN controller. They can cause various symptoms, such as reduced engine performance, transmission shifting problems, ABS warning light, etc.

If you own a car, you may have encountered a situation where your dashboard displays a warning light or a message indicating a problem with your vehicle. This is usually accompanied by a diagnostic trouble code (DTC) that helps you identify the source of the issue. DTCs are alphanumeric codes that are stored in the vehicle’s onboard computer system when a fault is detected.

One of the most common types of DTCs are related to the controller area network (CAN) bus, which is a communication system that connects various electronic control units (ECUs) in your car. The CAN bus allows the ECUs to exchange information and commands without a central computer. For example, the CAN bus can transmit data from the engine ECU to the transmission ECU, or from the anti-lock braking system (ABS) ECU to the instrument cluster.

However, sometimes the CAN bus can malfunction due to various reasons, such as wiring problems, faulty connectors, damaged ECUs, or external interference. When this happens, the communication between the ECUs can be disrupted or corrupted, leading to performance issues or system failures. To alert you of these problems, the CAN bus can generate different DTCs depending on the nature and location of the fault.

In this article, we will explain what are DTC error codes U0003, U0004, U0005, U0006, U0007, and U0008, what are their causes and symptoms, and how to troubleshoot and fix them.

What is a High Speed CAN Bus?

Before we dive into the details of each DTC error code, let’s first understand what is a high speed CAN bus and how it works.

A high speed CAN bus is a type of CAN bus that operates at a speed of up to 1 Mbps (megabits per second). It is used for critical applications that require fast and reliable data transmission, such as engine control, transmission control, ABS control, airbag deployment, etc.

A high speed CAN bus uses two dedicated wires for communication: CAN high and CAN low. These wires carry differential signals that represent binary data (0s and 1s). When the CAN bus is in idle mode, both wires carry 2.5 volts. When data bits are being transmitted, the CAN high wire goes to 3.75 volts and the CAN low wire drops to 1.25 volts, thereby generating a 2.5 volt differential between the wires. This differential signal is more resistant to noise and interference than a single-ended signal.

The high speed CAN bus follows a standard protocol that defines how data is formatted, transmitted, received, and acknowledged by the ECUs. Each ECU has a unique identifier that determines its priority on the bus. The higher the priority, the lower the identifier value. For example, an ECU with an identifier of 0x001 has a higher priority than an ECU with an identifier of 0x100.

The data on the high speed CAN bus is organized into frames, which are packets of information that contain an identifier, a data length code (DLC), a data field (up to 8 bytes), and a cyclic redundancy check (CRC) for error detection. Each frame begins with a start-of-frame (SOF) bit and ends with an end-of-frame (EOF) bit. Between frames, there is an inter-frame space (IFS) of at least 3 bits.

The ECUs on the high speed CAN bus communicate by sending and receiving frames according to an arbitration process. The arbitration process ensures that only one ECU can transmit at a time and that the highest priority frame wins the bus access. The arbitration process works as follows:

1. When an ECU wants to transmit a frame, it waits until the bus is idle and then sends its identifier bit by bit.
2. Each bit is monitored by all other ECUs on the bus. If an ECU detects a dominant bit (0) when it wants to send a recessive bit (1), it stops transmitting and goes into receive mode. This means that another ECU with a higher priority identifier is also transmitting.
3. The ECU that wins the arbitration continues to send its DLC, data field, and CRC until the end of the frame.
4. After sending the CRC, the transmitting ECU sends a recessive bit called CRC delimiter and waits for an acknowledgment from any other ECU on the bus.
5. If at least one other ECU has received the frame without errors, it sends a dominant bit called ACK bit. The transmitting ECU detects this ACK bit and completes the transmission.
6. If no other ECU sends an ACK bit, it means that either no other ECU was listening or there was an error in the transmission. The transmitting ECU detects this as a negative acknowledgment (NAK) and tries to retransmit the frame later.

What Do These DTC Error Codes Mean

Now that we have a basic understanding of how the high speed CAN bus works, let’s look at the meaning and causes of the DTC error codes related to it.

DTC error codes U0003, U0004, U0005, U0006, U0007, and U0008 are generic OBD-II codes that indicate a problem with the high speed CAN bus wires or the CAN controller. They are usually accompanied by other specific codes that point to the affected ECU or system.

The meaning of each code is as follows:

• U0003: High Speed CAN Communication Bus (+) Open. This code means that there is an open circuit in the CAN high wire, which prevents the communication between the ECUs.
• U0004: High Speed CAN Communication Bus (+) Low. This code means that there is a low voltage condition in the CAN high wire, which may be caused by a short to ground, a short to another wire, or a faulty ECU.
• U0005: High Speed CAN Communication Bus (+) High. This code means that there is a high voltage condition in the CAN high wire, which may be caused by a short to power, a short to another wire, or a faulty ECU.
• U0006: High Speed CAN Communication Bus (-) Open. This code means that there is an open circuit in the CAN low wire, which prevents the communication between the ECUs.
• U0007: High Speed CAN Communication Bus (-) Low. This code means that there is a low voltage condition in the CAN low wire, which may be caused by a short to ground, a short to another wire, or a faulty ECU.
• U0008: High Speed CAN Communication Bus (-) High. This code means that there is a high voltage condition in the CAN low wire, which may be caused by a short to power, a short to another wire, or a faulty ECU.

Symptoms of These DTC Error Codes

Depending on the severity and location of the fault, the symptoms of DTC error codes U0003, U0004, U0005, U0006, U0007, and U0008 may vary. Some of the common symptoms are:

• Check engine light on or flashing
• Reduced engine performance or power
• Transmission shifting problems or limp mode
• ABS warning light on or malfunctioning
• Airbag warning light on or malfunctioning
• Instrument cluster warning lights or gauges not working
• No communication with scan tool or diagnostic device

Troubleshooting

To troubleshoot and fix DTC error codes U0003, U0004, U0005, U0006, U0007, and U0008, you will need a scan tool or a multimeter to test the voltage and resistance of the CAN bus wires and connectors. You will also need a wiring diagram of your vehicle’s CAN bus system to locate the components and connections involved.

The following steps are general guidelines for diagnosing and repairing these codes. You may need to refer to your vehicle’s specific service manual for more details and instructions.

1. Connect your scan tool to your vehicle’s OBD-II port and retrieve all stored codes and freeze frame data. Note down any other codes related to the affected system or ECU.
2. Clear the codes and test drive your vehicle to see if they return. If they do not return, it may indicate an intermittent problem that is hard to reproduce. If they do return, proceed to the next step.
3. Locate the CAN bus wires and connectors under your dashboard or near your fuse box. Inspect them for any signs of damage, corrosion, loose connections, or bent pins. Repair or replace any faulty components as needed.
4. Using your multimeter set to DC volts, measure the voltage of the CAN high and CAN low wires at different points along the bus. You can use a wiring diagram to find the appropriate terminals or pins to probe. The voltage should be around 2.5 volts when the bus is idle and vary between 1.25 volts and 3.75 volts when data is being transmitted. If you find any abnormal readings (such as 0 volts or 12 volts), it indicates a problem with the wiring or the ECU.
5. Using your multimeter set to ohms (resistance), measure the resistance of the CAN high and CAN low wires at different points along the bus. You can use a wiring diagram to find the appropriate terminals or pins to probe. The resistance should be around 60 ohms when the bus is terminated and infinite when the bus is open. If you find any abnormal readings (such as 0 ohms or very low ohms), it indicates a problem with the wiring or the ECU.
6. If you have identified the faulty wire or ECU, replace or repair it as needed. If you are not sure which ECU is causing the problem, you can try to isolate it by disconnecting each ECU one by one and checking the voltage and resistance of the CAN bus wires. The ECU that restores the normal readings when disconnected is likely the culprit.
7. After you have fixed the problem, clear the codes with your scan tool and test drive your vehicle to verify that the codes do not return.

Conclusion

DTC error codes U0003, U0004, U0005, U0006, U0007, and U0008 are common codes that indicate a problem with the high speed CAN bus wires or the CAN controller. They can cause various symptoms, such as reduced engine performance, transmission shifting problems, ABS warning light, etc. To diagnose and fix these codes, you will need to follow a systematic troubleshooting procedure that involves scanning, inspecting, testing, and replacing the components involved. By following this article, you should be able to diagnose and solve these codes with ease.

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