P0B00 — AUXILIARY Transmission Fluid Pump Motor Phase U Current #

Note: The wording of this code may vary by manufacturer. Always refer to your vehicle’s OEM (Original Equipment Manufacturer) documentation for precise terminology.

• Toyota/Lexus: “AUXILIARY Transmission Fluid Pump Motor Phase U Current”
• General Motors (GM): “Auxiliary Transmission Fluid Pump Motor Phase U Control Circuit”
• Ford: “Aux Transmission Fluid Pump Motor Current Phase U Fault”

Summary of Code P0B00 #

The P0B00 diagnostic trouble code (DTC) points to an irregularity in the current measured through the Phase U winding of the auxiliary transmission fluid pump motor. This auxiliary pump is an electrically driven component responsible for maintaining proper transmission fluid pressure, especially during situations like engine start/stop events or when extra hydraulic pressure is needed. The vehicle’s transmission control module (TCM) or hybrid control module (HCM) continuously monitors the current in the motor’s windings (commonly called phases: U, V, and W) for faults, comparing them to expected values. If the current through Phase U is too high, too low, or otherwise abnormal, the module will trigger the P0B00 code. Left unchecked, this issue can affect transmission performance and longevity.

Common Symptoms for Code P0B00 #

Drivers and technicians may notice a range of symptoms when this code is active. These issues often relate to transmission operation and may vary in severity depending on the vehicle’s design and how the auxiliary pump is used. Watch for the following:

• Transmission warning light (check engine or hybrid system warning)
• Delayed or harsh shifting between gears
• Reduced transmission performance or limp mode activation
• Increased transmission fluid temperature (if the pump fails to engage)
• Unusual whining or buzzing noises from the transmission area

Probable Causes of Code P0B00 (Ranked by Likelihood) #

When diagnosing code P0B00, it’s crucial to focus on the most common root causes first. These range from electrical issues to faulty pump components, and some causes may overlap with other transmission or hybrid system faults.

1. Faulty auxiliary transmission fluid pump motor (winding or internal failure in Phase U)
2. Open or short circuit in the Phase U wiring harness or connectors
3. Corroded or loose electrical connections at the pump or control module
4. Faulty transmission control module (TCM) or hybrid control module (HCM)
5. Poor ground connection for the pump circuitry
6. Water intrusion or contamination of the pump or wiring

Step-by-Step Diagnostic for P0B00 #

A methodical approach ensures you identify the true source of the problem without replacing parts unnecessarily. Follow these steps:

• 1. Verify the code: Use a scan tool to confirm P0B00 is active and check for related codes.
• 2. Visual inspection: Examine the auxiliary pump wiring harness and connectors, especially at the Phase U terminal, for signs of damage, corrosion, or moisture.
• 3. Check electrical connections: Ensure all connectors are clean, tight, and free of corrosion, particularly at the pump, control module, and ground points.
• 4. Test wiring continuity: Using a multimeter, test the continuity and resistance of the Phase U circuit from the control module to the pump.
• 5. Check for shorts: Inspect for short-to-ground or short-to-voltage conditions in the Phase U circuit.
• 6. Actuate the pump: If possible, use a scan tool to command the auxiliary pump on while monitoring current flow in Phase U.
• 7. Inspect the pump motor: If wiring tests pass, bench-test the auxiliary pump motor for internal faults in the Phase U winding.
• 8. Evaluate the control module: As a last resort, test the TCM/HCM outputs and replace the module if all other components are verified good.

Possible Repairs for Code P0B00 #

Repairs should directly address the root cause identified during diagnostics. Avoid “shotgun” part replacement—targeted fixes are best:

• Replace the auxiliary transmission fluid pump motor if the Phase U winding is faulty
• Repair or replace damaged wiring in the Phase U circuit
• Clean and secure electrical connectors at the pump, module, and ground points
• Replace the TCM/HCM if it is confirmed to be malfunctioning
• Address any water intrusion by drying and protecting the affected area

Can You Drive With Code P0B00? #

Driving with P0B00 is generally NOT recommended. Because the auxiliary pump helps maintain critical transmission fluid pressure, a failure can lead to erratic shifting, overheating, or even transmission damage. Some vehicles may enter limp mode to protect the transmission, severely limiting performance. If you must drive, do so only for a short distance to reach a service facility, and avoid heavy acceleration or towing.

Vehicles Most Commonly Affected by P0B00 #

This code appears most frequently in hybrid and automatic transmission vehicles equipped with an electrically driven auxiliary transmission fluid pump. The following makes and models have reported P0B00, though the code can appear in other vehicles with similar systems:

• Toyota Prius (especially Generation 2 and 3 hybrids)
• Lexus RX, ES, and GS hybrids
• Chevrolet Volt
• Ford Fusion Hybrid
• Hyundai Sonata Hybrid
• Kia Optima Hybrid

Common Mistakes & Best Practices for P0B00 #

Technicians and DIYers sometimes make avoidable errors when troubleshooting this code. To save time and money:

• Don’t replace the pump motor before verifying wiring integrity and connector condition.
• Avoid overlooking ground connections; a poor ground can trigger current faults.
• Don’t ignore the possibility of water intrusion in hybrid vehicles parked outdoors.
• Always clear codes and retest after repairs to confirm resolution.
• Use OEM wiring diagrams for accurate pinout and circuit tracing.

Express FAQ: P0B00 #

Quick answers to common questions about this code:

• Q: Does P0B00 mean my transmission is about to fail?
  A: Not necessarily. The code points to a specific electrical fault with the auxiliary pump motor circuit, but acting quickly can prevent further transmission damage.
• Q: Can I just reset the code and keep driving?
  A: Simply clearing the code will not fix the underlying issue. The code is likely to return, and driving with a faulty auxiliary pump risks long-term damage.