Network, generic

U0084 Vehicle Communication Bus F (+) High

Check engine lightUsually off
EmissionsNot relevant
Drive cycleNot required
Limp modeNo

U0084 is set when the (+) wire of Vehicle Communication Bus F carries too much voltage, typically after contact with battery power. For a FlexRay Bus F, as in Audi's A6/A7, the wire is bus plus (BP); NXP rates the bus pins of its TJA1081B to withstand shorts to 14, 24 and 48 V and caps the output current at about 72 mA, so the hardware usually survives while signal levels are wrong. If Bus F is a CAN, CAN-High at supply voltage is one of the faults that may leave traffic running: onsemi still saw frames pass, and NXP counts it among hidden failures that only a diagnosing transceiver reports. Volvo lists shorts of CAN-High to power behind its truck code U008088. No manufacturer documents U0084 itself. Look for wiring touching a feed or a wet connector.

Likely causes

(+) wire of Bus F touching a battery or ignition feed
Water in a connector bridging the bus to a supply pin
Faulty aftermarket wiring tapped into the bus

Symptoms

  • Often no symptom, code found during a scan
  • Sporadic dropouts of functions on Bus F

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

U0068U0080U0083U0087

References: nxp.com, nxp.com, onsemi.com, static.nhtsa.gov.