Powertrain, generic · Hybrid propulsion

P0B09 Electric/Auxiliary Transmission Fluid Pump A Motor Supply Voltage Circuit/Open

Check engine lightOn
EmissionsRelevant
Drive cycleRequired
Limp modeNo

P0B09 concerns the power feed of auxiliary pump A's motor: the controller sees no valid supply voltage, usually because the circuit is open. GM documents it for its 2012-13 eAssist mild hybrids (BAS Plus): the hardware input reports an out-of-range voltage flag on the auxiliary pump circuit, enabled with ignition voltage above 9 V; failing for at least 0.375 s within a 1.2 s sample sets P0B09 as Type B, MIL after two trips. A related GM check, P2797, verifies during auto-stops that the pump draws power and that turbine speed does not flare on restart. A Hitachi patent on a battery-fed sub-oil pump shows the controller inhibits engine auto-stop when pump current behaves abnormally. Typical results: no auto-stop and a lamp. P0B0A/P0B0B are the low and high variants.

Likely causes

Blown fuse or open relay in the pump supply
Open supply wire or loose terminal at the pump connector
Poor ground connection of the pump motor
Voltage sensing input in the controller faulty

Symptoms

  • Engine auto-stop no longer occurs
  • MIL after two drive cycles
  • Shift flare or delayed drive engagement on restart

Repair estimate

DifficultyMediumDIY possible
Parts and labour€20–900
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

P0B0AP0B0BP2796P2797P0C23

References: gsi.ext.gm.com, patents.google.com, law.cornell.edu.