U0077

Code

Generic U — Network/User

Control Module Communication Bus E Off

Brand: Generic

Type: U — Network/User

Views: UK: 40 EN: 64 RU: 54

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Page language: EN

Physical damage to Bus E wiring (short to ground, short to battery, open circuit)
Failed control module or node on Bus E causing bus-off condition
Corroded/loose connectors or water intrusion at modules or junctions
Faulty termination resistor or missing termination at bus ends
Battery/charging system faults or low voltage causing modules to reset
Intermittent wiring fault (chafing, pinched harness)

Use a scan tool to read all modules and note which modules report loss of communication and any other related U-codes
Check freeze frame and occurrence data for conditions (ignition state, voltage)
Visually inspect Bus E wiring, connectors, junction boxes, and module grounds for corrosion, damage, or loose pins
Verify battery voltage and charging system operation during tests
Measure resistance between CAN_H and CAN_L at key access points to check termination (~60 Ω for a properly terminated CAN with two 120 Ω resistors)
Backprobe CAN_H and CAN_L (or LIN signal and ground if LIN bus) with a scope or multimeter to verify idle voltages and activity

CAN bus idle (recessive) typical: CAN_H ≈ 2.5 V, CAN_L ≈ 2.5 V
CAN bus dominant: CAN_H ≈ 3.5 V, CAN_L ≈ 1.5 V (differential ≈ 2.0 V)
Typical CAN network DC resistance between CAN_H and CAN_L across network ≈ 60 Ω (two 120 Ω terminators in parallel)
LIN idle: LIN signal ≈ 12 V (or system battery level) pulled low to transmit; check manufacturer spec for LIN levels

Record all codes and module responses with a professional scan tool; note which modules are unresponsive
Verify battery voltage (12.4–12.8 V at rest) and charging system under load; correct low-voltage conditions first
Perform a visual inspection of Bus E wiring, connectors, harness routing, and module mounting for damage, chafing, corrosion, or water ingress
Measure resistance between CAN_H and CAN_L at a convenient connector with ignition off; expect ~60 Ω for a healthy CAN bus. Large deviation indicates missing/failed terminator or short
With ignition on, measure CAN_H and CAN_L idle voltages at a module connector. Compare to expected values. Use an oscilloscope to confirm proper CAN activity and look for stuck dominant or noise
If a single module is suspected, disconnect that module (or isolate branches) and see if the bus returns to normal; a bus that recovers after disconnecting a node indicates that node is likely faulty
Wiggle test connectors and harness while monitoring bus signals for intermittent faults
Inspect or replace corroded connectors, repair damaged wiring with proper splice/reseal methods, and reinstall correct termination resistors if missing or incorrect
If wiring and connectors check good, consider replacing or reprogramming the suspected module only after confirming it is the cause (swap with known good if feasible)
Clear codes and retest to confirm repair; monitor for reoccurrence and document test results