Code
U0046
MERCEDES-BENZ
U — Network/User
Vehicle Communication Bus C
Views:
UK: 18
EN: 32
RU: 14
AI status
Completed
Completed
100%
Causes
- Open or short in CAN/LIN wiring for Bus C
- Failed CAN/LIN transceiver in one or more control modules
- Missing or incorrect termination resistor(s) on the bus
- Corroded/damaged connectors or poor ground/power to a node
- High bus load or electrical noise from aftermarket equipment
- Faulty module (gateway/module hardware) or incorrect/old software calibration
Symptoms
- One or more control modules report "no communication" or set related U-codes
- Warning lights and messages for systems on Bus C (may include comfort, gateway, ADAS or body functions)
- Intermittent or permanent loss of functions controlled by modules on Bus C
- Possible battery drain or inability to perform module programming if gateway unresponsive
What to check
- Read all stored and pending DTCs from all control modules and note freeze-frame/occurrence data
- Identify which modules report lost messages and whether multiple modules report Bus C faults
- Perform a visual inspection of wiring, connectors and grounding points along Bus C trunk and at modules
- With vehicle key on (engine off) measure idle voltages on CAN_H and CAN_L for Bus C at a connector near the gateway
- Measure termination resistance across CAN_H and CAN_L (typical ~60 Ω for two 120 Ω terminators in parallel)
- Check for shorts to battery voltage or ground on CAN_H/CAN_L (resistance and voltage tests)
Signal parameters
- Typical CAN recessive voltage: CAN_H ≈ 2.5 V, CAN_L ≈ 2.5 V (vehicle dependent)
- Typical CAN dominant voltages: CAN_H ≈ 3.5 V, CAN_L ≈ 1.5 V during active frames (approximate)
- Differential voltage during dominant state ≈ 2.0 V ±0.5 V
- Termination resistance across CAN_H and CAN_L ≈ 60 Ω (two 120 Ω terminators in parallel)
- Bus data rate varies by vehicle and network (e.g., 125/250/500 kbps or other - check manufacturer spec)
Diagnostic algorithm
- Record all U-codes and freeze-frame info from every module and note which modules are offline or reporting Bus C faults.
- Check vehicle service information for the topology and pinouts of Bus C (which modules are on this bus, bus length, termination location, data rate).
- Visually inspect harnesses, splice points and connectors for damage, corrosion or water ingress; repair obvious issues.
- With key on, engine off, measure CAN_H and CAN_L voltages at gateway and at end-node connectors; compare to expected values from service data.
- Measure termination resistance across CAN_H/CAN_L to confirm ~60 Ω; if out-of-range, locate and test terminators or open circuits.
- Check for shorts to 12 V or ground (measure resistance to battery + and - with connectors unplugged).
- Disconnect suspect modules one at a time (or isolate network segments via service connector) to identify a module that causes the bus to fail when connected (watch for bus voltage normalization).
- If a specific module is suspected, verify module power and ground, inspect connector pins, and bench-test or replace module transceiver if available.
- Check for software/calibration updates or known network bugs and reflash/update gateway and affected modules per manufacturer guidance.
- After repairs, clear codes, perform full communication check and road test to confirm no re-occurrence. If intermittent, perform extended monitoring with scope/trace log.
- Safety note: avoid probing live circuits with power applied in ways that could short CAN lines; follow vehicle manufacturer safety procedures.
Likely causes
- Damaged harness or connector at a module on Bus C
- Faulty CAN transceiver inside a control module
- Loss of bus termination (open or shorted resistor)
- Module losing power or ground causing communication loss
Fault status
Status
Bus C communication fault — messages missing or invalid. One or more modules on Bus C are not communicating normally.
Repair difficulty: Medium
Diagnostic time: 1-3 hours
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