Home›Trouble Codes›U0042

U0042 — Vehicle Communication Bus B Open

Generic OBD-II · all makesNetwork / communication

U0042 means the engine or network diagnostic system has detected an open circuit on Vehicle Communication Bus B, one of the data networks that links control modules. An open condition interrupts the electrical path so one or more modules cannot send or receive messages, often producing communication-related codes and warning lamps. The fault is usually in the bus wiring, connectors, or termination, not the main computer.

Symptoms

What causes it — most likely first

1. Open or damaged Bus B wiring/connectors

A break, chafe, corrosion, bent/pushed-out pin, or loose connector in the Bus B CAN high/low pair is the most common cause of an open-bus fault.

2. Failed or missing network termination resistor

Bus B typically uses 120 ohm terminating resistors at each end of the network; an open resistor changes bus impedance and can set an open-circuit code.

3. Defective module transceiver or disconnected module

A control module connected to Bus B with an internal CAN transceiver failure, or one that is unplugged, can interrupt bus continuity and prevent communication.

4. Poor power or ground to a Bus B module

If a module loses reliable power or ground, it may drop off the bus; however this usually sets module-specific communication codes before a general bus-open code.

5. PCM/ECM internal network failure

If the PCM is a gateway or terminator for Bus B, an internal CAN circuit fault could set U0042, but this is uncommon and should only be considered after wiring, termination, and other modules are ruled out.

How it's diagnosed

  1. Record all stored codes and identify which modules respond and which do not on the scan tool.
  2. Visually inspect Bus B harnesses, junction connectors, and module connectors for damage, corrosion, moisture, and pushed-out pins.
  3. Measure resistance across the Bus B CAN pair at an accessible point; with both 120 ohm terminators in place the total should be near 60 ohms, and near 120 ohms if one terminator is open.
  4. Check continuity of each Bus B wire between modules and check for shorts to power, ground, or the other CAN wire.
  5. Disconnect Bus B modules one at a time while monitoring bus resistance/voltage to isolate a module with an open or shorted transceiver; verify the suspect module’s power and ground before condemning it.

Code facts & related codes

SystemNetwork / communication
StandardizationSAE generic — identical on every make
Related codesU0040 — Vehicle Communication Bus B LowU0041 — Vehicle Communication Bus B HighU0043 — Vehicle Communication Bus B LowU0044 — Vehicle Communication Bus B HighU0045 — Vehicle Communication Bus B Shorted U0046 — Vehicle Communication Bus C

Frequently asked

Should I replace the PCM for U0042?

No, not usually. PCM replacement is rarely the correct fix for a Bus B open code. Start with the wiring harness, connectors, and termination resistors. Suspect the PCM only after confirming good power, ground, and bus integrity and isolating the PCM as the failed node.

Can I keep driving with U0042?

The vehicle may still run if the powertrain modules remain connected, but warning lights and disabled systems are common. It should be diagnosed promptly because safety-related modules may be offline.

Is Bus B the same on every vehicle?

No. The exact modules, wire colors, and speed of Bus B vary by manufacturer and model. Use the correct communication network diagram for the specific vehicle.

Can a low or dead battery cause U0042?

Low system voltage can cause many communication errors, but a U0042 open-bus code typically points to a physical break, termination fault, or module problem rather than just a weak battery.