OBD-II trouble code
U0074: Control Module Communication Bus B Off
A control module counted so many communication errors on the second bus (Bus B) that it shut itself off the network — the 'bus off' state. Almost always a wiring short or a failed module transceiver on that bus.
Quick facts
- System
- Network
- Category
- Network Communication
- Severity
- High severity
- Drivable
- No — stop driving until repaired
- Repair cost range
- $100 – $1,200
- DIY difficulty
- Shop recommended
What does U0074 mean?
U0074 means a control module has entered the CAN 'bus off' state on communication bus B — the vehicle's second network. CAN modules keep two internal error counters, and when a module accumulates too many transmit errors, the protocol forces it to disconnect itself from the bus entirely as a self-protection measure. This prevents one malfunctioning node from flooding the network with errors and taking everyone on that bus down with it. U0074 is the code that records a module had to take itself offline on Bus B.
Reaching the bus off state takes a serious, repeated fault — not occasional noise. The most common triggers are a hard short on the Bus B wiring (a bus wire shorted to power, to ground, or to its partner), a complete open in the bus, a failed terminating resistor on a high-speed segment, or a module whose CAN transceiver has failed and can no longer transmit valid frames. Unlike a soft performance fault such as U0029, bus off is a definitive event: the module gave up after its error count crossed the limit.
The symptoms are usually severe for whatever the second bus controls. Depending on which module went bus off and what rides Bus B on that vehicle, you can see disabled chassis or body systems, dead displays, or a no-start if a critical module shares that bus, along with a cascade of warning lights as other modules lose the data they expected. U0074 is a tow-it-in code in most cases — diagnosis centers on finding the wiring short or the failed module that drove the error counter to its limit.
Common causes
- A Bus B wire shorted to power or ground
- Bus B wires shorted together
- Complete open (break) in a Bus B wire
- Failed CAN transceiver inside a module on the second bus
- Terminating resistor lost on a high-speed Bus B segment
- Severe connector corrosion or water intrusion on Bus B
- Damaged harness from rodent activity, accident, or chafing
- A defective module continuously transmitting errors before going offline
Symptoms
- One or more systems on the second bus completely non-functional
- Multiple warning lights illuminated at once
- Scan tool cannot communicate with the affected module at all
- Displays or gauges dead or frozen
- No-start or stall if a critical module rides Bus B on that vehicle
- Loss of comfort, body, or chassis features tied to the second bus
- Bus off / lost-communication U-codes stored in several modules
Diagnostic steps
- 1.Scan every module and note which report communication faults; confirm which module went bus off and that Bus B is the affected network.
- 2.Use a wiring diagram to identify the Bus B circuits and the modules that share them on the specific vehicle.
- 3.With the battery disconnected, measure resistance across the Bus B pair; about 60 ohms is healthy on a terminated high-speed segment, near 0 indicates a short, infinite an open, ~120 ohms a lost terminator or branch.
- 4.Inspect the Bus B wiring end to end for shorts to power/ground, breaks, chafing, and rodent damage.
- 5.Check connectors and splice packs for corrosion, water intrusion, and backed-out terminals.
- 6.If wiring checks out, disconnect modules on that bus one at a time to identify a module with a failed transceiver driving the bus off.
Repair cost
$100 – $1,200
Diagnosis typically runs $150-$300. A wiring short or open repair lands at $150-$600 depending on where the damage is and how hard it is to reach. A failed terminator or connector repair is on the lower end. Replacing a module with a failed transceiver, including programming, runs $400-$1,000. European and luxury vehicles with multiple buses tend toward the upper range.
Estimate your repair
Run the numbers for your vehicle
Open the Repair Cost Estimator with module communication / can bus diagnosis preselected. Adjust labor rate and vehicle category to fit your situation.
DIY vs shop
Leave this one to a qualified shop. It typically involves emissions-critical components, refrigerant handling, or other work that requires manufacturer-grade tooling, training, or certification. DIY attempts often produce a more expensive problem than the original code.