What Does P0B09 Mean?
The powertrain control module has detected a fault in the supply voltage circuit feeding the auxiliary transmission fluid pump motor, indicating an open circuit or general electrical problem in the power supply path. Without proper supply voltage, the pump motor cannot operate at all, completely eliminating auxiliary transmission fluid flow during engine-off hybrid modes. This is a fundamental power delivery failure to the pump system.
Common Causes
30%
Blown fuse or fusible link in the auxiliary pump supply circuit
25%
Open or corroded supply voltage wiring between battery/junction box and pump controller
20%
Failed pump supply relay not closing to provide battery voltage
15%
Poor ground connection for the pump motor control module
10%
Auxiliary pump control module internal power supply failure
Diagnostic Steps
1
Check the auxiliary pump fuse in the under-hood fuse box. If blown, check for short circuits before replacing. Inspect the fusible link in the supply wire if applicable.
2
Measure battery voltage at the pump control module supply pin with the ignition on. If no voltage is present, trace the supply circuit from the fuse box to the controller looking for open connections.
3
Test the auxiliary pump supply relay by swapping with an identical relay from a non-critical circuit. Listen for relay click when the ignition cycles or use the scan tool to command relay operation.
4
Check the ground connection for the pump control module. Measure resistance from the module ground pin to the battery negative terminal — should be less than 0.5 ohms. Clean and tighten the ground bolt.
5
If supply voltage is present at the module but the pump still does not operate, the module's internal power supply circuitry may have failed. Check for any module-specific DTCs and verify CAN communication is active.
Estimated Repair Cost
$50 - $1,200
Parts + labor, varies by vehicle and location
The powertrain control module has detected a fault in the supply voltage circuit feeding the auxiliary transmission fluid pump motor, indicating an open circuit or general electrical problem in the power supply path. Without proper supply voltage, the pump motor cannot operate at all, completely eli...
The most common cause of P0B09 (Auxiliary Transmission Fluid Pump Motor Supply Voltage Circuit/Open) is: Blown fuse or fusible link in the auxiliary pump supply circuit
Typical repair costs for P0B09 range from $50 to $1,200, depending on the vehicle, location, and whether you do it yourself or go to a shop.
The auxiliary pump will be completely non-functional without supply voltage. Auto-stop and EV mode will be disabled. Engine-on driving is safe, but the transmission relies entirely on the mechanical pump. Diagnose the supply circuit promptly — the fix may be as simple as a blown fuse, but the root cause of the fuse failure must be identified.
Start by connecting an OBD2 scanner to read the code and any freeze frame data. Then follow the diagnostic steps specific to P0B09 to identify the root cause.
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Quick Info
Category
Powertrain
System
Hybrid Transmission Auxiliary Systems
Difficulty
Type
Generic (SAE)
Recommended Tools
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