U0001 Code – What Does It Mean & How To Fix It

This is one of the most frequent OBD2 trouble codes. Read the full article below to know what it means, how to fix it, and what other codes may show related to it.

Definition

High Speed CAN Communication Bus

Meaning 

When the OBDII scanner pulls up a Code U0001, it means there is an issue with the High Speed Communication Area Network (CAN) Bus.

Modern vehicles use a network of internal onboard computers to run the various systems like suspension, traction control, brakes, the engine, transmission and more. 

These computers, called modules, “talk” via the controller area network (CAN) bus. Bus is the term for internal networks that handle input and output data to and from multiple connected computers.

With vehicle computers like those found in your car, truck or SUV, the CAN bus consists of two lines: CAN High and CAN Low. It’s important to note some manufacturers call them by other names like CAN C and CAN IHS, but they serve the same function regardless.

CAN High operates at a data rate of 500 kilobits/second and CAN Low transfers data at 125 kilobits/second. These connect to gateway modules that process and transfer messages between the CAN buses.

If the check engine light comes on and you see a Code U0001,  it’s indicating a problem with the CAN High bus

Causes

Generally there are only a couple reasons you see this error code pop up on your OBDII scanner.

  • A faulty control module is causing communication errors
  • A problem exists between CAN bus connections

Symptoms

Here are commons symptoms you might see your car exhibit if you pull a U0001 trouble code.

  • An illuminated check engine light
  • A secondary code specifying a failed module (always helpful when trying to track down the problem)
  • Performance problems ranging from the vehicle not starting to the air conditioning not working, depending on which module is unable to communicate with the rest of the computers in your car’s network.

Diagnosis

Finding the source of this trouble code can be a bit tricky, as U0001 pops up intermittently or occurs due to low battery voltage or a dead battery.

Here’s the step-by-step process our technicians use to diagnose the problem correctly.

  • First, we clear the code and see if it pops up again.
  • If the code alert recurs, we make a close visual inspection of the CAN bus and all connections. A trained technician is best qualified to check for more obvious issues like broken wires and loose connections.
  • If a problem exists with the wiring or connectors, our technician will make the repair and see if the code comes up again.
  • Conversely, if nothing is discovered by visual inspection, we check manufacturer technical service bulletins (TSBs) for that vehicle’s year, make and model.  Generally, TSBs recommend specific diagnostic and repair procedures from manufacturer feedback submitted by their service centers. If we find a related TSB for your vehicle, it can greatly reduce diagnostic time since it’s a known issue.

Common Diagnostic Mistakes

It’s pretty difficult to go wrong diagnosing this problem.

The only mistake an automotive service technician can make is failing to check for TSBs related to your vehicle’s trouble code.

How Serious is a U0001 Trouble Code?

If your vehicle’s AC or other important system has stopped working, you want to resolve this issue as soon as possible. It doesn’t mean that the vehicle is necessarily unsafe to drive, but you want to get it checked out as soon as you can.

We’ve found that code U0001 may be accompanied by a 2-character sub-code that provides additional information about specific module failure or CAN bus component failure. These subcodes make diagnosis easier because they indicate problems like an open or short to ground.

What repairs can fix the code?

  • Repair/replace faulty/broken wiring CAN bus wiring
  • Repairing/replacing faulty CAN bus connectors
  • Replacing failed or burnt out control modules for the non-functioning system

Related Codes

  • 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.

Conclusion

Seeing a U0001 code come up on your trouble code scanner means your vehicle has a computer controller issue and it needs to be tracked down and repaired or replaced.

A highly qualified technician with access to a TSB database should perform a system diagnostic, and the problem can generally be resolved quickly after they find the specific cause of your trouble code.

Ignoring this trouble code is not recommended, as it is often the first warning sign of a more complex and expensive problem that may cause your vehicle to break down in the future. Get your vehicle checked out by a qualified technician as soon as you can.