Code
U0011
MERCEDES-BENZ
U — Network/User
Medium Speed CAN Communication Bus Performance
Views:
UK: 16
EN: 23
RU: 15
AI status
Completed
Completed
100%
Causes
- Damaged or chafed CAN wiring harness
- Corroded/loose CAN connectors or module pins
- Incorrect or missing termination resistor(s) on the medium-speed CAN ring
- Short to battery or ground on CAN_H or CAN_L
- Faulty module(s) driving the medium-speed CAN
- Faulty or intermittent power or ground to one or more network modules
Symptoms
- One or multiple body/control modules not responding to scan tool (doors, windows, HVAC, instrument cluster, gateway)
- Intermittent or complete loss of specific body functions (e.g., central locking, lighting, mirrors)
- Multiple related U-codes or CAN faults stored
- Erratic behavior of systems on the medium-speed network
- No communication with certain modules during diagnostics
- Possible battery drain if a module is stuck awake or shorted
What to check
- Use a diagnostic scan tool to list all network modules and note which are not responding
- Record freeze-frame and related network DTCs (look for additional U-codes)
- Visually inspect wiring harnesses, junction boxes, and connectors for damage, corrosion, or water intrusion
- Disconnect aftermarket accessories that connect to the bus and re-test
- Measure DC voltage on CAN_H, CAN_L and to vehicle ground with ignition ON (see signal parameters)
- Measure resistance between CAN_H and CAN_L with ignition off (expected ~60Ω for a properly terminated bus)
Signal parameters
- Idle single‑ended voltages: CAN_H ≈ 2.5 V, CAN_L ≈ 2.5 V (each ~2.5 V when recessive)
- Dominant state voltages: CAN_H ≈ 3.5 V, CAN_L ≈ 1.5 V (differential ~2.0 V)
- Differential voltage (dominant) ≈ 2.0 V; recessive differential ≈ 0 V
- Bus termination: combined resistance between CAN_H and CAN_L ≈ 60 Ω (two 120 Ω terminators in parallel common on some networks)
- Typical medium-speed CAN bit rates vary by model—commonly 125 kbps or 250 kbps; verify factory spec for the specific Mercedes model
Diagnostic algorithm
- Connect a factory-capable or advanced scan tool. Read and document all stored network DTCs and which modules respond on the medium-speed CAN.
- Note timing and conditions when the fault occurs (ignition state, accessory use, weather).
- Visually inspect harnesses, junction boxes and connectors for damage, corrosion or water intrusion in areas between modules on the medium-speed CAN. Repair any obvious wiring/connector damage.
- Check module power/ground circuits for proper voltage and ground integrity; a module with bad supply can corrupt the bus.
- With ignition OFF, measure resistance between CAN_H and CAN_L at a convenient access point — expect ~60 Ω. If not, locate missing or extra terminator.
- With ignition ON, measure single-ended voltages on CAN_H and CAN_L relative to chassis ground; compare to expected values in signal_params.
- Use an oscilloscope or CAN sniffer to capture waveforms while actuating affected systems; look for missing frames, collisions, excessive error frames, or a stuck dominant level.
- Isolate the fault by disconnecting suspected modules one at a time (start with modules added recently or those that fail to respond) and observe if bus performance improves.
- Remove aftermarket devices or known low-quality modules from the bus and retest.
- Repair wiring or connectors as found; replace any confirmed faulty module. Reprogram/configure replacement modules per factory procedure.
- Clear codes, cycle ignition, and road/test to confirm repair. If intermittent persists, monitor bus during the operating condition when the fault appears.
Likely causes
- Open or shorted CAN_H or CAN_L wiring at a connector or harness damage
- Corroded or loose connector at a control module on the medium-speed bus
- Missing/failed termination resistor or wrong resistance value
- Faulty module (one module pulling the bus low/high intermittently)
- Power/ground fault to a key body control module causing bus failure
Fault status
Status
Medium-speed CAN bus communication degraded or interrupted. One or more modules on the medium-speed network are not communicating or bus signals are outside expected parameters.
Repair difficulty: Medium
Diagnostic time: 1.0-3.0 hours
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