U0074: Control Module Communication Bus "B" Off
A control unit has switched off its connection to communication bus B because of too many errors.
High risk
Have it checked soon. Several systems can stop working at once.
Short answer · Can I keep driving with U0074?
Often yes, with care, but warning lights and functions may come and go. Have the network checked soon.
What does U0074 mean?
U0074 means a module stopped talking on bus B to protect the network. A short circuit in the wiring, a faulty module or water in a connector are typical causes.
What the car does when the code is set
Several modules may lose communication at once. Depending on which modules are affected, warning lights come on, gauges or displays drop out, the engine may not start or the car may enter limp mode.
How the system works
Modern cars connect their control modules through data networks, most often CAN (Controller Area Network). Modules exchange messages many times per second. If messages stop or the network signal is disturbed, modules store U codes describing the communication problem.
Common symptoms
Possible causes
Listed without ranking. The cause on your car depends on make, engine and mileage — see the maker notes below.
- Low or unstable battery voltageModules drop off the network when voltage dips, for example with a weak battery or during cranking.
- Damaged or wet wiringChafed, corroded or water-damaged network wires and connectors interrupt messages.
- A faulty module pulling the bus downOne failing module can disturb communication for all the others.
- Missing or faulty terminating resistorCAN networks need termination at both ends; a fault changes the signal.
- Aftermarket accessoriesPoorly installed alarms, trackers or audio can interfere with the network.
How it is diagnosed
- Check the battery and charging firstMeasure battery voltage at rest and with the engine running. Many network codes disappear once voltage is stable.
- Read codes from all modulesA full-system scan shows which modules report lost communication and which one is silent.
- Inspect connectors and wiringLook for corrosion, water entry and damaged wires, especially near the battery, under seats and in door hinges.
- Measure the networkA workshop can measure CAN resistance and signals at the diagnostic connector to locate the fault.Do not probe connector pins with sharp tools; damaged terminals cause new faults.
Questions about U0074
Why do so many warning lights come on with U0074?
Control units share data over the network. When the network fails, each of them stores its own fault and switches on its warning light.
Can a weak battery cause network codes?
Yes. Low voltage is one of the most common causes of U codes, especially stored codes that appear after starting.