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U0401 – What Does It Mean and 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.


U0401 is an OBD-II Code that refers to Invalid Data Received From ECM/PCM


A U0401 code is a diagnostic trouble code that occurs when the other control modules on the vehicle are not communicating with each other. In this instance the Controller Area Network bus communications, or simply CAN indicates there’s a loss of communication with the Engine Control Module (ECM) or Powertrain Control Module (PCM). This network provides information about the car’s operation in real time, as well as instructions for changes to its functioning.

The PCM (Powertrain Control Module) not only serves as a data-handling center, but it also monitors the information exchange system between the modules to ensure that it is functioning properly. If there is a communication failure between two or more of these modules for an extended period of time, the fault code U0401 OBDII will be set.


Common causes for this code include:

  • Abnormal system voltages
  • Faulty Engine Control Module (ECM)/Powertrain Control Module (PCM)
  • ECM/PCM circuit poor electrical connection
  • ECM/PCM harness is open or shorted
  • Multiple bad sensors going of at once or in conjunction with bad modules


The symptoms of a U0401 code are:

  • Illuminated Check Engine Light 
  • Random power loss
  • Sounds that imitate misfires 
  • In the event of a catastrophic failure, may enter a fail-safe or limp mode
  • ABS or other safety systems such as Traction Control and/or Stability Control might be deactivated or unavailable, depending on the manufacturer.


To diagnose a U0401 DTC code, a technician would:

  1. Scan for codes in the ECM and look at the freeze frame data for failure.
  2. Visually inspect the wiring harness for damage.
  3. Check the system voltage and compare to normal readings.
  4. Verify the ECM/PCM is getting power and ground.
  5. Check the communication circuit between modules.
  6. Test components in the CAN bus. 
  7. Reset the code and test drive the vehicle.
  8. If the code returns, further diagnosis will be required.

Common mistakes

The following are some of the most common mistakes that a technician may make when diagnosing a U0401 code:

  • Not thoroughly inspecting all the wiring and connectors.
  • Not checking the system voltage.
  • Not verifying that the ECM/PCM is getting power and ground.
  • Not testing all components in the CAN bus.
  • Reinstalling modules and sensors without verifying proper function.
  • Assuming that the issue is with a specific module or sensor.

How serious is this?

A U0401 code is a serious code and should be addressed as soon as possible. There may be a loss of power, engine performance, or safety features depending on the severity of the issue.

What repairs can fix the code?

The following are solutions that may fix this problem:

  • Repair or replace any faulty parts such as wiring, connectors, or control modules
  • Check all system voltages and correct as necessary
  • Clean or replace the ECM/PCM if it is not getting power or ground
  • Check for any other codes that may be related 

Related codes

A U0401 is related to and may be accompanied by the following codes:

P0100 – Mass or Volume Air Flow Circuit Malfunction

P0105 – MAP Sensor Circuit Malfunction

P0107 – Barometric Pressure Circuit Low

P0606 – ECM/PCM Internal Control Module Memory Check Sum Error

U0300 – Loss of Serial Data Communication with ECM/PCM


In conclusion, the U0401 code is a diagnostic trouble code that refers to a problem with the communication between modules on the Controller Area Network (CAN) bus. The code is serious and should be addressed as soon as possible. There may be a loss of power, engine performance, or safety features depending on the severity of the issue. The most common causes of this code are abnormal system voltages, faulty control modules, bad electrical connections, or shorted or open wiring harnesses. The code can usually be reset and the vehicle tested to see if it returns. If it does, further diagnosis will be required.