U0086

Code

Generic U — Network/User

Vehicle Communication Bus F - Low

Brand: Generic

Type: U — Network/User

Views: UK: 31 EN: 42 RU: 35

AI status

Completed

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Page language: EN

Causes

Short to ground on the Bus F low conductor
Open, high-resistance or chafed wiring connection in Bus F
Failed module actively pulling the low line to ground
Missing or incorrect termination (open or damaged resistor)
Poor battery voltage or engine/vehicle ground integrity affecting bus transceivers
Damaged connector or corrosion allowing low resistance to ground

Symptoms

One or more control modules on Bus F not responding or reporting errors
Multiple related systems inoperative or degraded (depending on modules on Bus F)
Intermittent communication or sporadic warning lights
Difficult-to-start or cranking issues if powertrain module is on that bus
Stored U-codes related to network communication and module loss

What to check

Scan all modules; note all U-codes and which modules are not present on Bus F
Visually inspect wiring harnesses, connectors, and junctions along Bus F for damage, corrosion or water
Verify battery voltage and main chassis/body grounds are clean and tight
Measure voltage on the Bus F low conductor with key ON, engine OFF (see signal_params for expected values)
Measure termination resistance across Bus F high and low (expected ≈60 ohms for CAN with two 120 Ω terminators)
Use an oscilloscope to view CAN bus waveforms (check recessive/dominant levels and noise)

Signal parameters

Typical CAN bus recessive: CAN_H ≈ 2.5 V, CAN_L ≈ 2.5 V (differential ≈ 0 V)
Typical CAN bus dominant: CAN_H ≈ 3.5 V, CAN_L ≈ 1.5 V (differential ≈ 2.0 V)
A Bus low voltage stuck below ~1.8–2.0 V on recessive may indicate a short to ground
Termination resistance across CAN_H and CAN_L: ≈ 60 Ω (two 120 Ω terminators in parallel)
Differential amplitude: ≈ 1.5–2.5 V peak-to-peak for a healthy CAN bus

Diagnostic algorithm

Safety: Park vehicle, key OFF, wear eye protection. Record all stored network DTCs and freeze frames with a scan tool.
Replicate: Turn ignition ON (engine OFF). Confirm U0086 and any related U-codes are present.
Visual inspection: Inspect Bus F wiring, connectors, junction blocks and modules for damage, corrosion or water intrusion.
Battery/ground: Check battery voltage and main ground straps. Repair poor battery/ground connections before further testing.
Static voltage check: With key ON, measure CAN_L (Bus F low) to chassis ground and CAN_H to chassis ground. Compare to expected values in signal_params.
Termination check: With ignition OFF, measure resistance across CAN_H and CAN_L at a convenient connector—expect ≈60 Ω. If open or very low, investigate terminators.
Oscilloscope: Use a scope to observe bus waveforms while a module transmits. Look for bus stuck low, clipped signals, excessive noise, or lack of differential amplitude.
Isolate modules: If bus low is present, disconnect modules or non-OEM devices on Bus F one at a time (power and data connectors) to find a change. A module removal that restores bus voltage points to a faulty module.
Repair wiring/connectors: Repair any shorted/open wiring, replace corroded connectors, and restore correct termination and grounds.
Replace suspected module last: If a specific module removal restores the bus, bench-check or replace the suspected faulty module per OEM procedures.
Clear codes and retest: Clear codes, cycle ignition, and road test (or exercise network) to verify the fault does not return.
If fault persists: Escalate to manufacturer-specific diagnostics; consult wiring diagrams for Bus F layout and splice locations.

Likely causes

Shorted Bus F low line to chassis ground (most likely if low voltage measured on low line)
Failed transceiver in a module on Bus F pulling the line low
Open/shorted termination resistors (wrong resistance or open circuit)
Power/ground fault to modules on the bus causing abnormal transceiver behaviour
Water intrusion or corrosion at a common connector/junction on Bus F

Fault status

⚠️ Status

Bus F low-voltage line detected below expected threshold; communication on Bus F is degraded or lost. Investigate wiring, termination, module transceivers, and power/ground systems.

🟡 Repair difficulty: Medium

⏱️ Diagnostic time: 1-4 hours