U0001 OBD-II: High Speed CAN Communication Bus

What does code U0001 mean?

Code U0001 stands for High Speed Communication Area Network (CAN) Bus

Modern vehicles have many computers on board. These computers (referred to as modules) communicate with one another via the controller area network (CAN) bus.

The CAN bus consists of two lines: CAN High and CAN Low. Depending on the manufacturer, they may be referred to by other names, such as CAN C and CAN IHS. Regardless, CAN High has a data rate of 500k bits/second, whereas CAN Low has a data rate of 125k bits/second. A gateway module is used to process and transfer messages between the CAN buses.

Code U0001 indicates there is a problem with the CAN High bus.

U0001 symptoms

• An illuminated check engine light
• A secondary code specifying a failed module
• Performance problems ranging from the vehicle not starting to the air conditioning not working, depending on which module is unable to communicate

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Common causes for U0001

Code U0001 is typically caused by one of the following:

• A faulty control module
• A problem with the CAN bus

How to diagnose and repair U0001

Perform a preliminary inspection

Sometimes U0001 can pop up intermittently, or it can result from a dead battery. Clear the code and see if it returns. If it does, the next step is to perform a visual inspection. A trained eye can check for issues such as broken wires and loose connections. If a problem is found, the issue should be repaired and the code cleared. If nothing is discovered, check for technical service bulletins (TSBs). TSBs are recommended diagnostic and repair procedures put out by the vehicle manufacturer. Finding a related TSB can greatly reduce diagnostic time.

Check for a faulty control module

The next step is to check for a faulty control module. To do this, a technician will first check if there are any other diagnostic trouble codes (DTCs) stored. Module specific DTCs may indicate a problem with that particular module. For example, code U0101 may indicate a problem with the TCM.

Then a roll call of the modules can be performed. This is done with a diagnostic scan tool connected to the vehicle’s diagnostic port. Once the tool is hooked up to the vehicle, it can communicate on the network as if it were another control module. The tool will address the modules individually, making sure they all respond. A module that doesn't answer indicates a potential problem with that module or its circuitry.

Finally, a technician may go through and unplug the modules one at time, while monitoring the network. If unplugging a certain module restores network communication, there’s a problem with that module or its circuitry. With code U0001, it’s a good idea to start with the modules on the CAN High circuit first, since this code pertains to that side of the network.

A non-functional module should have its circuit checked prior to replacement. Like any other electrical device, a control module must have good power and ground. The module's software should also be checked. Many times, a module can be reprogrammed instead of replaced.

Check the network

At this point, the network itself will need to be inspected. A professional will typically start network testing at the vehicle data link connector. The connector has 16 pins – pin 6 is CAN High and pin 14 is CAN Low. A digital multimeter (DMM) can be connected to one of more of these pins for initial network inspection. Then testing will move to various connections and points throughout the CAN network.

The two terminating resistors can also be checked by connecting a DMM between pins 6 and 14 of the data link connector. If the resistors are intact, the DMM should read 60 ohms. A breakout box can also be used to check network integrity. This tool plugs directly into the diagnostic port, where it is used to monitor network communication and function.

Other diagnostic codes related to U0001

• U0002: Code U0002 indicates the CAN High bus has a performance problem.
• U0003: Code U0003 indicates the CAN High bus (+) has an open circuit.
• U0004: Code U0004 indicates the CAN High bus (+) has a low signal.
• U0005: Code U0005 indicates the CAN High bus (+) has a high signal.
• U0006: Code U0006 indicates the CAN High bus (-) has an open circuit.
• U0007: Code U0007 indicates the CAN High bus (-) has a low signal.
• U0008: Code U0008 indicates the CAN High bus (+) has a high signal.

Note: All the 'U' codes are network communication codes.

Code U0001 technical details

In some cases, code U0001 may be accompanied by a 2-character sub-code. This code provides additional information that can make diagnosis easier. For example, the sub-code might indicate whether the failure is an open or short to ground.