U0401 OBD-II: Invalid Data Received From ECM/PCM "A"

What does code U0401 mean?

Code U0401 stands for Invalid Data Received from ECM/PCM.

The engine control module (or powertrain control module) is the computer than manages engine operation. The engine control module (ECM) receives input from sensors throughout the vehicle. It then uses that information to powertrain-specific outputs, such as the fuel injectors or coil packs.

In modern vehicles, the ECM communicates with other modules via the Controller Area Network (CAN) bus. When it comes to older vehicles, modules typically communicate via a more primitive, serial data network. This is partially because older vehicles have a limited number of modules onboard.

The CAN Network consists of two lines called CAN High and CAN Low. Can High transmits data at a rate of 500k bits/second, whereas CAN Low transmits data at a rate of 125k bits/second. There are two terminating resistors at the ends of the CAN bus.

Code U0401 indicates one or more modules onboard the vehicle has received invalid data from the ECM.

U0401 symptoms

• Illuminated warning lights
• ECM-related performance issues

Common causes for U0401

Code U0401 is typically caused by one of the following:

• A dead battery
• ECM problems
• A problem with the CAN bus

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How to diagnose and repair U0401

Perform a preliminary inspection

Sometimes U0401 can pop up intermittently, or it can result from a dead battery. This is especially true if the code is a history code and not current. 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 the battery

The ECM needs proper voltage to operate. Before doing anything else, the battery and charging system should be checked for proper operation and repaired as needed. Then, clear the codes and see if they return.

Check for other DTCs

Additional diagnostic trouble codes (DTCs) may indicate problems elsewhere that are affecting ECM operation. For example, multiple communication DTCs may indicate a problem with the CAN network. Any additional DTCs should be addressed prior to diagnosing U0401.

In the case where multiple communication DTCs are stored, diagnosis will shift to the CAN bus. Like any other electrical circuit, the bus can be checked for problems such as opens and shorts. Bus testing typically starts at the data link connector, either with a digital multimeter (DMM) or a breakout box. Pin 6 of the datalink connector is CAN High, whereas pin 14 is CAN low. If a problem is detected, further testing and repair of the CAN bus can be competed as needed.

Check for a faulty control module

If U0401 is the only DTC stored, the ECM module itself should be checked. The easiest was to start this process is by attempting to communicate with the ECM using a diagnostic scan tool. Once connected to the vehicle, the tool acts like just another module on the network. It can be used to address the ECM module. If the module does not respond, there is a problem with it.

Before condemning the module, it's important to check its circuit. Like any other electronic device, the ECM module must have proper power and ground. This can be checked using a DMM.

If the module's circuit it good, yet it still won't communicate, it is likely faulty. Before replacing the module, however, its software should be checked. Often times a module can be reprogrammed instead of replaced.

Other diagnostic codes related to U0401

All the 'U' codes are network communication codes. Codes U0100 to U0300 are lost communication with XX module codes.