U0071

Code

Generic U — Network/User

Vehicle Communication Bus E (-) High

Brand: Generic

Type: U — Network/User

Views: UK: 33 EN: 31 RU: 32

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Page language: EN

Causes

Short to battery or ignition power on the Bus E negative line (CAN_L) or related wiring
Open or high-resistance connection on the Bus E positive or negative conductor
Failed CAN transceiver or ECU/module with a shorted internal driver
Missing or incorrect termination (open/shorted resistor) on Bus E
Corroded/wet/poor connectors or damaged harness insulation contacting a voltage source
Aftermarket device or module tied into Bus E incorrectly

Symptoms

Multiple control modules report communication faults or go into limp/fail-safe
Loss of functions controlled by modules on Bus E (e.g., instrument cluster, HVAC, ABS — depends on vehicle)
Illuminated MIL or multiple warning lamps
Intermittent or constant diagnostic trouble codes related to module communication
Slow or unresponsive networked features (locks, displays, gateways)

What to check

Read all stored and pending U- and P-codes; note which modules report loss of communication
Visually inspect Bus E wiring, connectors, and modules for damage, corrosion, or water ingress
With ignition ON (engine off), measure DC voltage of Bus E lines to chassis ground at multiple points
Measure resistance across the bus terminals (with ignition and battery disconnected) to check termination (~60 ohm for CAN with two 120Ω resistors)
Use a lab scope or CAN bus analyzer to view differential waveform and noise on Bus E
Disconnect aftermarket modules or recently serviced components that tie into the bus and re-test

Signal parameters

Expected idle voltages (typical CAN): CAN_H ≈ 2.5 V, CAN_L ≈ 2.5 V (both ~2.5 V when recessive)
Dominant state voltages (typical): CAN_H ≈ 3.5 V, CAN_L ≈ 1.5 V
Abnormal condition for U0071: CAN_L higher than expected (e.g., > ~2.8–3.0 V) or both lines stuck near battery voltage
Termination resistance: approximately 60 Ω across CAN_H and CAN_L (two 120 Ω ends in parallel)
Signal shape: differential square/voltage transitions with clean edges; excessive noise or level offset indicates fault

Diagnostic algorithm

Record vehicle information, freeze frame (if available) and all stored codes. Note which modules are reporting loss of communication.
Perform a visual inspection of Bus E harness, connectors, module ground points and nearby power sources for damage, corrosion, or fluid intrusion.
With ignition ON (engine off), measure voltage to chassis ground at the Bus E negative conductor (CAN_L) and Bus E positive conductor (CAN_H) at a convenient connector near a module. Compare to expected idle voltages (~2.5 V).
If CAN_L is high, trace wiring toward each module and junction. Unplug connectors one at a time while watching the voltage and stored codes to see if the level or fault clears (helps isolate offending module/branch).
With battery disconnected, measure resistance across CAN_H and CAN_L at convenient connector points to verify termination (~60 Ω). If open or very low, investigate termination resistors and wiring short.
Use an oscilloscope or CAN bus analyzer to inspect differential waveform while operating the network. Look for stuck dominant/recessive levels, reflections, excessive noise, or missing transitions.
If a suspect module is identified, perform connector and pin voltage checks with the module connected and then disconnected. Replace or bench-test the module transceiver per manufacturer procedure before replacement.
Repair damaged wiring, re-seat/clean connectors, replace failed modules or termination resistors as required. Reconnect everything and clear codes.
After repair, confirm proper bus voltages and waveform, then road test to ensure the fault does not return and all modules communicate normally.

Likely causes

Corroded/damaged connector or wiring causing short/high voltage on Bus E (-)
Failed CAN transceiver in one module pulling the negative line high
Loss or incorrect value of termination resistor(s) on Bus E
Short to battery voltage on the Bus E negative conductor
Recently installed accessory or repaired harness incorrectly connected to the bus