Diagnostic article Generic OBD-II
P0B09: Auxiliary transmission-pump supply relay feedback disagrees
P0B09 monitors the auxiliary transmission-fluid pump supply circuit through a dedicated feedback line.
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Documented application
2009 General Motors diagnostic parameter file 09OBDG08 Engine Diagnostics.pdf. Section-specific modules and calibrations; no per-code model or engine applicability established.
The code alone does not identify a failed part or establish that a vehicle is safe to drive. Confirm the vehicle, reporting module and applicable service procedure.
P0B09 concerns the auxiliary transmission-fluid pump's supply-relay circuit in GM's BAS hybrid section. The controller compares the relay-control output with an echo-back signal. A mismatch indicates an electrical supply-control problem rather than directly measuring pump flow or transmission pressure.
Supply control and PWM control are separate paths
The source places P2796 beside P0B09 for the pump's separate PWM control feedback. That distinction matters: a supply-relay fault and a modulation-command fault are not interchangeable merely because they involve the same pump.
P0B09 includes both high and low feedback disagreements relative to the commanded output. The documented check occurs while the module is awake and lists a 320-millisecond evaluation. A feedback voltage without its command state cannot reproduce that comparison.
Preserve the correct circuit identity
Record supply-command and feedback information, along with any separate pump-control codes. The code alone does not prove that the pump motor is failed or that a hydraulic repair is required. The exact vehicle's circuit diagram is needed to identify the monitored relay path.
Follow the appropriate hybrid and transmission service procedure rather than manually powering the pump from improvised connections. Verification should establish valid supply feedback and completed monitoring, plus the functional checks specified for the pump. A pump heard operating briefly or an empty code list after clearing does not prove that the supply-control path is reliable.
Source and applicability
2009 General Motors diagnostic parameter file 09OBDG08 Engine Diagnostics.pdf. Section-specific modules and calibrations; no per-code model or engine applicability established. This explanation describes the documented monitor, not a universal repair procedure.
Article source documents
Source checks describe how the text was prepared; they do not certify a diagnosis or confirm compatibility with every vehicle using this code.
Original code reference
Dictionary definition
Electric/Auxiliary Transmission Fluid Pump A Motor Supply Voltage Circuit/Open
This definition retains its original catalog scope. The article above is limited to its explicitly documented application.
Reference scope
Generic OBD-II definition.
A manufacturer label does not establish the model, year, engine or module to which a diagnosis applies. Read the safety and diagnostic limits.
Definition source
- DTC Database: pinned community code definitions
Source checked · MIT