U1000 OBD-II: Manufacturer Controlled DTC

What does code U1000 mean?

Code U1000 stands for Communication Area Network (CAN) Lost Communication

Modern vehicles use a data bus to allow communication between onboard computers. This data bus is called a communication area network (CAN) and the computers are referred to as modules. A typical late-model vehicle has many modules and they all share their information via the CAN bus. There are dedicated modules for most parts of the vehicle. For example, there’s an engine control module (ECM), transmission (TCM) and brake control module (BCM).

There are two communication lines on the CAN bus: CAN high (H-line) and CAN low (L-Line). The CAN H-Line has a dormant voltage of about 0.25-volts that rises to 0.65-volts when communicating. On the other hand, the CAN L-Line has a dormant voltage of around 11-volts that drops to 4.65-volts when communicating. The H-Line and L-Line wires are housed together in a twisted wiring harness.

When there’s a communication problem, the system will store a ‘U’ diagnostic trouble code (DTC). Code U1000 means one or more modules is not able to communicate properly over the CAN network.

U1000 symptoms

• An illuminated check engine light
• A secondary code specifying a failed module
• Performance problems ranging from a no-start condition to inoperative HVAC system, depending on which module is unable to communicate

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Common causes for U1000

Code U1000 is typically caused by one of the following:

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

How to diagnose and repair U1000

Perform a preliminary inspection

Sometimes U1000 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

After performing a preliminary inspection, a technician will check for a faulty control module. To do this, the technician will see if there are any other module specific DTCs stored. For example, there may also be a code U0101 stored, indicating a communication problem with the TCM.

If no other codes are found, a roll call of the modules may be performed. This is done using a diagnostic scan tool. The scan tool connects to the vehicle through the diagnostic port. Once connected, the tool acts like another module on the network. The scan tool performs a roll call, addressing the other modules individually. A module that doesn’t respond, indicates a 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.

NOTE: a non-communicating module doesn’t necessarily mean the module has failed. The module may not have proper power or ground. Or it may just need to be reprogrammed.

Check the network

If an isolated module doesn’t seem to be the problem, the network itself will have to be checked. This is often done using a digital multimeter (DMM). The DMM is connected between the two network pins at the data link connector.

There are two terminating resistors at each end of the CAN bus. If one of those resistors fails, the bus will still operate. However, if they both fail, the bus will typically shut down. A professional will check the integrity of these resistors by checking their resistance. To do this, a DMM (set to ohms) is connected to the diagnostic port. A normal reading should be approximately 60 ohms. The network is checked for shorts and opens in the same fashion.

A savvy technician may also test the network with a breakout box. A breakout box is a measurement tool used to test CAN communication signals and listen to network communication. The box is connected directly to the vehicle diagnostic port.

Other diagnostic codes related to U1000

• U1001: Code U1001 indicates the ECM is not sending or receiving CAN communication signals
• U1002: Code U1002 indicates the BCM is not sending or receiving CAN communication signals

Code U1000 technical details

U1000 is a manufacture specific code. Because of this, the code definition may very somewhat between vehicles.