Powertrain, generic · Hybrid propulsion

P0B0A Electric/Auxiliary Transmission Fluid Pump A Motor Supply Voltage Circuit Low

Check engine lightOn
EmissionsRelevant
Drive cycleRequired
Limp modeNo

P0B0A means the supply voltage measured at the motor of auxiliary pump A is lower than expected while the pump should be running. No manufacturer documents this exact code publicly; GM's eAssist tables only cover the open-circuit case as P0B09. A Mikuni patent shows why the controller measures this voltage at all: it combines motor current, supply voltage and speed to judge the hydraulic load, so a sagging supply also falsifies pump diagnosis. Causes are high resistance in the feed or ground, a weak 12 V battery, a failing DC/DC converter or a voltage drop when the pump starts. CARB requires circuit checks on such outputs. The pump runs slowly or not at all and start-stop is usually inhibited. P0B09 covers an open circuit, P0B0B a supply that is too high, P0562 low system voltage in general.

Likely causes

Corroded terminal or high resistance in the pump feed or ground
Weak 12 V battery or poor charging
DC/DC converter or alternator output too low (hybrid)
Voltage sensing input in the controller faulty

Symptoms

  • Start-stop function inactive
  • MIL or transmission warning
  • Jerky drive-off after an engine stop

Repair estimate

DifficultyMediumDIY possible
Parts and labour€20–700
Time0.5–3 h

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

Parts involved

aux trans pump a motorwiring harnesstcm

Related codes

P0B09P0B0BP2797P0562

References: patents.google.com, law.cornell.edu, patents.google.com.