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U1000 Code: What to Do If Your Car’s CAN System Malfunctions 

  • Cars Explained
  • Celine Jerly
  • 6 minutes

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 You might not easily notice symptoms of DTC U1000. But once detected, it’s best to get a proper fix at the first opportunity. The manufacturer-specific fault code indicates problems with your vehicle’s communication system. While it does not pose an immediate threat, a communication breakdown potentially affects the car’s performance over time. So, here’s all you need to know about fixing a U1000 code.   

What does a U1000 code indicate?  

U1000 is a manufacturer-specific OBD-2 Trouble Code applicable to GM, Infiniti, Nissan, and Isuzu models. The exact definition of this error code varies from manufacturer to manufacturer. But it generally indicates a communication system failure – specifically, defects in the car’s Controller Area Network (CAN) Data Bus System.   

Manufacturer-specific Definitions for U1000 

Vehicle Manufacturer 

DTC U1000 Definition 

GM  Class 2 Communication Malfunction Conditions 
Nissan  CAN Communication Circuit 
Infiniti  CAN Communication Line – Signal Malfunction 
Isuzu  Class 2 Communication ID Not Learned 

How serious is a DTC U1000?  

While a communication system failure is not as severe as other engine troubles, a U1000 code error leads to additional symptoms that affect engine performance and drivability features. However, the severity of this DTC varies for different makes and models. For example, the U1000 code for Nissan cars is clearly described as a potential threat to drivability, while other manufacturers deem it a less urgent issue.   

What is a CAN communication circuit?  

Your car has a communication system that relays data through complex electrical connections from one point to another. This communication interface is called the CAN-bus system and is essential for the vehicle’s optimal operation.   

CAN has two communication lines – H-line (High) and L-line (Low). Both lines are held together in a twisted wiring harness. When relaying data, the voltage of the CAN H-line rises from about 0.25-volts to 0.65-volts. At the same time, the CAN L-Line dormant voltage of around 11-volts drops to 4.65-volts. In today’s advanced systems, data is delivered through hertz signals, 5-volt references, 12-volt power, and system grounds.   

CAN (Controller Area Network) data bus system consists of multiple modules with specific functions. For example, the ECM (Engine Control Module) collects temperature/pressure sensor data to monitor engine performance. Based on ECM data, TCM (Transmission Control Module) controls shift timing, while BCM (Body Control Module) is in charge of illuminating lights on the instrument cluster. If these modules do not communicate as intended, the car can’t function properly.   

What happens when a car’s CAN-bus malfunctions?  

If one or more of the car’s communication system modules malfunctions, the Diagnostic Trouble Code (DTC) U1000 is triggered. The module(s) isn’t relaying data over the CAN-bus as expected. The communication could also happen due to faults in the wiring.   

When there is no proper communication within the CAN-bus, it can adversely affect your car’s functions. You may not notice the effects on drivability and engine performance immediately, but you could experience power loss or engine trouble over time.  


What are the symptoms of a U1000 code error?  

In most cases, the only indication of a DTC U1000 is the Check Engine light. Additional symptoms like engine stalling, hesitation or failure to start the car, inoperable HVAC system, and/or lack of power could be experienced, depending on the specific cause of communication failure. OBD-2 U1000 is often accompanied by secondary codes that specify the defective module or circuit.   

What causes code U1000?  

The fault in the communication system that triggers a U1000 code varies for different vehicle makes and models. Generally, this DTC indicates a faulty control module or circuit. The secondary code helps pinpoint the specific cause.   

If there are no other active codes, faulty grounds, damaged/corroded wiring or a bad connector contact at the module junction/wiring harness could have triggered the U1000 code. The ECM might need a replacement to fix the code in some cases.   

How do I fix error code U1000?  

Once the U1000 code is detected on the OBD-2 scanner, a proper diagnosis is required to identify and fix the specific cause. If the diagnosis is wrong, you might end up with unnecessary expenses. Therefore, it is best to take your car to the shop if you do not have the necessary resources and experience. Replacing a module, for example, requires manufacturer-specific software for “flashing.”  

CAN-bus Communication System Diagnosis – Checklist  

The first step is to scan the code and record all other active DTC. Diagnosing the secondary code helps find the root cause of the communication system failure. For example, code U0101 indicated an issue with the TCM. Additionally, you can check the manufacturer’s technical service bulletins (TSBs) to understand the problem specific to your car’s make and model.  

  • Use factory wiring diagrams to inspect the connections and clear all ground connections to the CAN-bus system.   
  • Check wiring harnesses and module connections for corrosion or other damage.   
  • Once these check out, run the code again to see whether it persists.   
  • If the U1000 code returns after visual inspection of all connections, locate appropriate service literature that outlines the procedure to diagnose and isolate the fault in a module or connector.  
  • In some cases, a dead battery could also cause the U1000 code to pop up now and then.   

auto service electrician wiring car maintenance U1000

U1000 Code – System Repair Procedure  

All modules must be cleared once the code is scanned and a visual inspection is completed.   

If there are no secondary codes, the technician does a roll call of modules. First, a scan tool is connected to the vehicle through the diagnostic port. When connected, the scan tool acts as a module on the network and individually addresses all other modules. If a certain module fails to respond, it indicates a problem with the circuit or the module itself.   

The technician may also unplug each module to check whether the communication is restored. However, a non-communicating module does not always indicate that it failed. Instead, it might just need reprogramming or isn’t receiving enough power.   

If all the modules check out and the code U1000 returns, the network itself could be the problem. So a digital multimeter (DMM) is used to check the network by connecting it to the two network pins at the link connector.   

The two terminating resistors at each end of the CAN-bus are also checked for integrity. The system will still work if one fails, but it typically shuts down if both fail. The DMM is set to ohms and connected to the diagnostic port to check the resistance; 60 ohms is normal. Shorts and opens in the network are also checked in the same manner.   

How much does it cost to fix the U1000 code?  

If there are no replacements, you won’t have to spend much except the labor cost. But in case of a faulty module that cannot be repaired, expect to shell out more on fixing U1000. For example, an ECM replacement will cost you around $800. Labor costs and other fees are additional. Overall, depending on the shop you choose and the vehicle model, the cost of replacing the ECM can go as high as $2000! 

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