Network, generic

U0086 Vehicle Communication Bus F (-) Low

Check engine lightUsually off

EmissionsNot relevant

Drive cycleNot required

Limp modePossible

U0086 says the (-) wire of Vehicle Communication Bus F sits too low, usually shorted to ground. On a FlexRay Bus F, as in the Audi A6/A7, that wire is bus minus (BM). At rest both FlexRay lines float near half the transceiver supply, and NXP specifies a near-zero difference between them in idle. A grounded BM leaves BP well above it, a permanent positive difference that receivers read as DATA_1 instead of idle, which disturbs wake-up and frame recognition across the cluster. On a CAN the comparable fault, CAN-Low to ground, is usually harmless for traffic and stays hidden unless a transceiver with bus diagnosis reports it, as NXP describes. No manufacturer documents U0086 publicly. Measure the (-) wire to chassis with the battery disconnected.

Likely causes

(-) wire of Bus F chafed against body metal

Harness trapped under a seat rail or trim

Transceiver of one module pulling the (-) line to ground

Symptoms

Modules on Bus F fail to wake or drop out
On CAN layouts often no noticeable symptom

Repair estimate

DifficultyHardWorkshop job

Parts and labour€60–800

Time1–5 h

Typical range across vehicles; the real cost depends on what the testing finds.

Parts involved

bus f wiringcan transceiver

Related codes

U0070 U0080 U0085 U0087

References: nxp.com, nxp.com, law.cornell.edu.