OBD-II trouble code
U0028: Vehicle Communication Bus B
A general fault on the vehicle's second communication bus (Bus B) — a secondary CAN network used by modules that aren't on the main high-speed powertrain bus. Usually a wiring, connector, or single-module fault that disrupts 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 U0028 mean?
U0028 is a generic network code that means a fault has been detected on 'Bus B' — the vehicle's second communication bus — without naming one specific module as missing. Modern vehicles rarely have just one network. The high-speed CAN that links the engine and transmission controllers is one bus; manufacturers add additional buses (commonly labeled B, C, and beyond) to carry chassis, body, comfort, or infotainment traffic so the main powertrain bus isn't overloaded. U0028 says that second bus itself is sick rather than identifying the culprit.
Like any CAN segment, Bus B depends on a balanced pair of wires and, on high-speed segments, terminating resistors at each end. If a wire is shorted, broken, or chafed, if a connector corrodes, if a terminator fails, or if one module on that bus starts transmitting corrupted frames and 'jabbering,' the electrical balance collapses and the modules sharing the bus lose reliable communication. Because U0028 is generic, it tells you the bus is faulted rather than pointing at the exact wire or module — diagnosis is about isolating which segment or module is dragging that network down.
What the driver notices depends on which systems live on Bus B for that particular vehicle. It can range from a cluster of warning lights and disabled convenience features to a no-start if a critical module rides that bus. Several systems may drop into a fail-safe state at once, and a scan tool may struggle to reach the affected modules. Because the consequences and the exact bus assignment vary by manufacturer, U0028 is best diagnosed with a wiring diagram for the specific vehicle rather than by guessing at parts.
Common causes
- Short to power or ground on a Bus B circuit
- Bus B wires shorted together
- Open or broken Bus B wire (rodent damage, accident, chafing against a bracket)
- Failed terminating resistor on a high-speed Bus B segment
- Corroded or backed-out terminals at a module connector or splice pack
- Water intrusion in a connector or harness section carrying Bus B
- A single failed module on Bus B corrupting bus traffic
- Poor or loose ground supporting a module on the second bus
Symptoms
- Multiple warning lights illuminated at once
- Loss of features served by the second bus (comfort, chassis, or body systems)
- No-start or stall if a critical module rides Bus B on that vehicle
- Gauges or displays erratic, frozen, or dead
- Scan tool cannot communicate with one or more modules on Bus B
- Intermittent loss of features tied to road vibration
- Several communication (U-series) codes stored across modules
Diagnostic steps
- 1.Record all stored codes in every module first — the pattern of which modules report faults helps identify which bus and segment are involved.
- 2.Use a wiring diagram for the specific vehicle to confirm which modules and circuits make up Bus B, since the assignment varies by manufacturer.
- 3.With the key off and battery disconnected, measure resistance across the Bus B pair; on a terminated high-speed segment about 60 ohms is healthy, ~120 ohms suggests a missing terminator or branch, near 0 a short, and infinite an open.
- 4.Key on, check Bus B signal voltages against the service data for that network.
- 5.Inspect Bus B wiring and connectors along the harness for chafing, rodent damage, corrosion, and water intrusion.
- 6.Disconnect modules on that bus one at a time while watching communication; if the bus recovers when a specific module is unplugged, that module or its branch wiring is the fault.
Repair cost
$100 – $1,200
Diagnosis alone often runs $150-$300 because isolating a bus fault is methodical work. A wiring or connector repair typically lands at $150-$600 depending on access. A failed terminator or splice repair is on the lower end. Replacing a module that's corrupting the bus, including programming, runs $400-$1,000. Costs climb on European and luxury vehicles with more complex multi-bus architectures.
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.