Auxiliary Transmission Fluid Pump Motor Supply Voltage Circuit/Open
The P0B09 code indicates an open or short circuit in the supply voltage circuit for the auxiliary transmission fluid pump motor. This pump is typically found in hybrid or start-stop vehicles to maintain transmission fluid pressure when the engine is off. The issue may be due to a blown fuse, damaged wiring, faulty relay, or a defective pump motor. The powertrain control module (PCM) detects a voltage discrepancy in the circuit and sets this code.
1. Symptoms You Will Notice
- Transmission warning light illuminated
- Reduced fuel economy or transmission performance
- Transmission may not shift properly or go into limp mode
- Possible transmission overheating
2. Most Common Causes
- Blown fuse or faulty relay
- Damaged or corroded wiring/connectors
- Defective auxiliary transmission fluid pump motor
- Faulty PCM (rare)
3. Step-by-Step Diagnosis
Step 1: Scan for codes and record freeze frame data.
Step 2: Visually inspect the auxiliary pump fuse and relay; replace if blown.
Step 3: Check wiring and connectors for damage, corrosion, or loose connections.
Step 4: Test voltage supply at the pump motor connector with a multimeter (should be battery voltage with key on).
Step 5: If voltage is present, test pump motor resistance and operation; replace if faulty.
4. Frequently Asked Questions
Is it safe to drive with this code?▼
It is not recommended. The auxiliary pump is critical for maintaining transmission fluid pressure during engine stop-start events. Driving could lead to transmission damage or failure.
How much does it cost to fix?▼
Repair costs typically range from $150 to $400, depending on the cause. A simple fuse replacement may cost under $50, while replacing the pump motor can be $200-$400 including labor.
5. Commonly Affected Makes
This is a generic OBD2 code, potentially applicable to any manufacturer using this diagnostic standard.