Network, generic

U0085 Vehicle Communication Bus F (-) Open

Check engine lightUsually off
EmissionsNot relevant
Drive cycleNot required
Limp modePossible

U0085 points to a severed (-) conductor on Vehicle Communication Bus F, that is bus minus (BM) where Bus F is FlexRay, as in the Audi A6/A7, or CAN-Low in a CAN. How hard the break hits depends on the topology. FlexRay may run two independent channels, and Wikipedia notes that traffic can carry on with reduced bandwidth when one of them fails, so a break on one channel may cost little more than redundancy. In a single-channel layout every node beyond the break goes silent. Transceivers cannot see an open wire directly, as NXP explains for CAN; the module infers it from missing frames. Volvo's truck bulletin KC-2079 also lists open CAN-Low as a trigger of U008088. No manufacturer documents U0085 itself. Check continuity of the (-) wire through every connector.

Likely causes

(-) wire of Bus F broken after chafing or repeated flexing
Spread or corroded terminal of the (-) wire
Poor earlier repair splice in the bus pair

Symptoms

  • Individual modules on Bus F stop responding
  • Code present with little or no effect when a second channel takes over

Repair estimate

DifficultyHardWorkshop job
Parts and labour€60–700
Time0.5–5 h

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

Parts involved

bus f wiringmodule connector

Related codes

U0069U0080U0082U0086

References: en.wikipedia.org, nxp.com, static.nhtsa.gov, law.cornell.edu.