U0140: Lost Communication With the Body Control Module

What U0140 means

U0140 is a generic OBD-II network code: Lost Communication With Body Control Module. A module other than the BCM sets it. That is often the engine or powertrain control module, the instrument cluster or a gateway. The code sets when that module expects regular messages from the BCM and they stop arriving within a set time window. The code does not mean the BCM is bad. It only says communication was lost. The cause can be the BCM itself, its power or ground supply, the network wiring between modules, or low system voltage that made several modules drop off the network at once.

The most common causes are: • a weak or discharged battery; • loose or corroded battery terminals; • a blown fuse feeding the BCM; • a poor ground connection; • damaged or corroded network wiring and connectors; • water getting into the BCM or its connectors; • less often, a failed or incorrectly programmed module; Symptoms depend on what the BCM controls on your vehicle. That can include interior and exterior lighting, power locks and windows, wipers, the security or immobilizer system, and sometimes cluster functions. A single stored U0140 after a battery replacement or jump start is not unusual and may never return. A code that keeps coming back or causes symptoms needs diagnosis.

How modules communicate with the BCM

Most modern vehicles connect their modules with a controller area network (CAN). Each module sends short messages onto a shared pair of twisted wires, usually called CAN high and CAN low. Each module also listens for the messages it needs from other modules. The BCM broadcasts information such as door status, lighting requests and key or security status, and other modules monitor those messages. If the messages disappear, the receiving module stores a U code naming the module it lost.

Every module needs stable power, a good ground and an intact connection to the network. The BCM can reset or stop transmitting while the rest of the network is fine if its supply voltage drops too low, its fuse opens, or its ground develops high resistance. If the network wires themselves are shorted, open or corroded, several modules may lose each other at the same time. In that case you will often see a group of U codes rather than U0140 alone. That pattern is one of the most useful clues. One lost module points toward that module's power, ground or branch wiring. Many lost modules point toward a shared cause such as low voltage or a network fault.

What changes the diagnosis

Network layout varies by manufacturer, model and year. On some vehicles the BCM sits on the high-speed CAN bus that also carries engine data. On others it sits on a slower body network or a single-wire network, or behind a gateway module that passes messages between networks. Some manufacturers combine the BCM with the fuse box under a different name, such as a smart junction box or integrated power module. Because of this, a test that works on one vehicle may not reach the BCM's network on another. An example is measuring resistance between the network pins at the diagnostic connector. The correct wiring diagram for your exact vehicle shows which circuits, fuses and connectors are involved.

When and how the code sets also matters: • Only during cranking: often points to low battery voltage or a voltage drop in the starting system. • After rain or a car wash: suggests water reaching a connector or the module. • Right after installing an accessory such as a remote starter, alarm or trailer wiring: points toward that installation, especially if it taps into BCM circuits. • With a battery that keeps going flat: the code may be a result of the voltage problem rather than its cause, and a module that fails to go to sleep can itself create parasitic draw.

Hybrids and EVs: 12-volt versus high voltage

Hybrids and electric vehicles still run the BCM and other control modules from a 12-volt auxiliary battery, separate from the high-voltage traction battery. On these vehicles the 12-volt battery is usually charged by a DC-DC converter rather than an alternator. A weak 12-volt battery can cause U codes, warning messages and a no-ready condition even when the traction battery is charged. Owner checks for U0140 on these vehicles stop at the 12-volt system. Do not touch, unplug or probe orange high-voltage cables, the traction battery or its service disconnect. Those require the manufacturer's isolation procedure, protective equipment and training. Some hybrids and EVs also require a specific sequence before you disconnect the 12-volt battery, so check the owner's manual first.

Checks an owner can do

Start with the battery and its connections, because low or unstable system voltage is a common and inexpensive cause. With the vehicle off and rested, a healthy, fully charged 12-volt battery typically reads about 12.6 volts. A noticeably lower reading suggests the battery is discharged or weak. With the engine running on a conventional vehicle, charging output usually falls roughly between 13.5 and 14.7 volts, though specifications vary. Inspect the battery terminals for looseness, corrosion or a cracked clamp. Check that the main ground cables to the body and engine are tight and clean. A load test at a parts store or shop tells you more than a resting voltage reading alone.

Next, use the owner's manual or fuse box lid to find the fuses that feed the BCM, and check them. Replace a blown fuse only with one of the same fuse rating. A fuse that blows again means there is a short that needs tracing, not that you need a larger fuse. Look for signs of water, such as wet carpet, dampness near the BCM location, a leaking cowl drain or a blocked sunroof drain. If an accessory was recently installed, consider whether its connections could have disturbed BCM wiring.

Read all stored codes with a scan tool, not only U0140, and note which modules report them. Record whether each code is current or history. Then clear the codes and see whether U0140 returns under the same conditions. If you are comfortable with a multimeter, measure voltage drop between the negative battery post and the engine block while cranking. This can reveal poor ground connections that a visual check misses. Avoid unplugging modules or probing network pins at random. Probing the wrong connector can damage terminals or create new faults.

When to get vehicle-specific help

A generic code description cannot tell you which module on your vehicle stored U0140, which network the BCM uses, or how its power and ground circuits are routed. Those answers come from the factory service information for your exact vehicle. That includes the wiring diagram, connector locations and the manufacturer's diagnostic procedure. Some manufacturers also publish technical service bulletins or software updates for network codes. A replacement BCM often needs programming or security setup before it works, so swapping the module on a guess is rarely a good first step.

Have the vehicle diagnosed by a shop if any of these apply: • The battery, charging output, fuses and grounds check out, but the code keeps returning. • The vehicle will not start. • Lighting or locks behave erratically. • Multiple network codes are stored. Choose a shop with a capable scan tool and access to service information. A technician can check network activity and termination, measure supply and ground at the BCM connector, and find whether the fault is in the wiring, a connector or the module itself. On hybrids and EVs, use a shop qualified for high-voltage work if diagnosis goes beyond the 12-volt system.