U0140 OBD-II: Lost Communication With Body Control Module

Code U0140 stands for Lost Communication with Body Control Module (BCM)

What does code U0140 mean?

On a vehicle, the body control module (BCM) is the computer responsible for various body-related functions. The BCM receives input from sensors and switches throughout the vehicle. It then uses this information to determine control of body-related outputs. For example, the BCM may receive input from the power window switch when it's depressed. In turn, the BCM will send power to the window motor, lowering the window.

The BCM communicates with other onboard computers (referred to as modules) over the controller area network (CAN) bus. Two lines make up the CAN bus: CAN High and CAN low. CAN High communicates at a rate of 500k bits/second, whereas CAN Low communicates at 125k bits/second. There are two terminating resistors at each end of the CAN bus. These resistors are used to terminate communication signals, since data on the bus flows both ways.

On some vehicles, the BCM acts as a gateway module, enabling communication between the CAN High and CAN Low buses. It may also perform other duties, such as acting as an interface to the anti-theft system and climate control.

Code U0140 indicates the BCM is not receiving or transmitting messages on the CAN bus.

U0140 symptoms

• Illuminated warning lights
• BCM-related performance issues

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Common causes for U0140

Code U0140 is typically caused by one of the following:

• A dead battery
• A faulty BCM
• A problem with BCM circuit
• A problem with the CAN bus

How to diagnose and repair U0140

Perform a preliminary inspection

Sometimes U0140 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

Proper battery voltage is essential to BCM operation. Before proceeding any further, the battery should be checked and recharged/replaced as needed. Then, clear the code and see if it returns.

Check for other DTCs

Additional diagnostic trouble codes (DTCs) may indicate problems elsewhere that could be affecting BCM operation. For example, DTCs set for multiple modules may indicate a problem with the CAN network. Any additional DTCs should be addressed prior to delving into U0140.

In the case where multiple modules DTCs are stored, diagnosis will shift to the CAN bus. The bus can be checked for typical circuit malfunctions, including shorts and opens. This process usually starts at the data link connector. The data link connector has 16 pins - pins 6 and 14 are CAN high and CAN low. A technician will connect a digital multimeter (DMM) to one or both of these pins for testing. If a problem is indicated, further testing can be performed at other parts of the CAN network.

CAN bus operation can also be checked with a breakout box. This tool hooks up directly to the data link connector. It is used to monitor bus operation and detect problems.

The two CAN bus terminating resistors can be check with a DMM at the data link connector. This is done with the DMM connected between pins 6 and 14 of the connector. A reading of 60 ohms indicates the resistors are intact.

Check for a faulty control module

If there are no other DTCs stored, the BCM itself should be checked. Typically, the first thing a technician will do is try to communicate with the BCM. This is done using a diagnostic scan tool, connected to the vehicle diagnostic port. Once connected to the vehicle, the scan tool can communicate directly with the vehicle's modules, including the BCM.

A non-responsive BCM must be diagnosed. Before condemning the BCM, its circuit should be checked with a DMM. Like any other electrical device, the BCM must have proper power and ground.

If the circuit is OK, the BCM is probably the problem. However, before replacing the BCM, its software should be checked. Often times the BCM can be reprogrammed instead of replaced. If software isn't the issue the BCM will need to be replaced. Often times, the BCM will need to be reprogrammed after replacement.

Other diagnostic codes related to U0140

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

Code U0140 technical details

On many vehicles, battery voltage must be between 9 - 16 volts for code U0140 to set.