Network, generic

U012F Lost Communication With Generator B Control Module

Check engine lightUsually off
EmissionsNot relevant
Drive cycleNot required
Limp modeNo

U012F is reserved for vehicles with a second generator, labelled B, whose regulator or control electronics talk to the powertrain or hybrid controller over a bus. That can be a second alternator on heavy-duty trucks or a further motor-generator in a hybrid drive. No manufacturer explains U012F in an open document; the closest documented case is the A unit under U012D, where Ford's smart alternator hangs on a LIN line and loses contact through wet connectors or chafed wires. CARB's network monitoring rules apply in the same way. If generator B goes quiet, the controller can no longer set its output, so charging power drops and the battery or high-voltage system may run short. U0120 covers a starter generator.

Likely causes

Wet or corroded connector in the bus line to generator B
Fuse, power or ground of the generator B electronics missing
Bus wire chafed against engine or bracket
Generator B regulator or control module faulty

Symptoms

  • Charging system warning
  • Battery voltage falls during driving
  • Hybrid system warning or reduced electric drive on hybrids

Repair estimate

DifficultyMediumWorkshop job
Parts and labour€60–1,500
Time0.5–4 h

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

Parts involved

generator b modulecan bus wiring

Related codes

U012DU0120

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