Auxiliary Transmission Fluid Pump Motor Phase U Current Low
The P0B01 code indicates that the powertrain control module (PCM) has detected a low current condition in phase U of the auxiliary transmission fluid pump motor. This pump is responsible for circulating transmission fluid to cool and lubricate the transmission, especially during start-stop events or when the engine is off. A low current reading suggests an electrical fault, such as a failing motor, damaged wiring, or a poor connection, which can lead to inadequate fluid circulation and potential transmission damage.
1. Symptoms You Will Notice
- Check Engine Light illuminated
- Transmission overheating or slipping
- Reduced fuel economy
- Transmission warning messages on dashboard
2. Most Common Causes
- Open circuit in the phase U wiring between the inverter/controller and the pump motor
- Short to ground in the phase U circuit
- Open or high resistance in the phase U winding of the auxiliary pump motor
- Loose, corroded, or damaged terminals in the pump motor connector
- Internal failure of the power driver (MOSFET/IGBT) in the pump control module or TCM
- Mechanical pump failure preventing free rotation (seizure) — though typically causes high current, severe binding can alter the expected current profile
3. Step-by-Step Diagnosis
Step 1: Scan for additional codes and record freeze frame data.
Step 2: Visually inspect the auxiliary pump wiring and connector for damage, corrosion, or loose pins.
Step 3: Use a multimeter to measure resistance and voltage at the pump connector phase U pin; compare to specifications.
Step 4: Test the pump motor by applying direct battery voltage to verify operation; if no response, replace pump.
Step 5: If wiring and pump are okay, check PCM output signals; consider PCM replacement if all else fails.
4. Frequently Asked Questions
Is it safe to drive with this code?▼
It is not recommended. Low current in the auxiliary pump can lead to insufficient transmission cooling and lubrication, potentially causing severe transmission damage over time.
How much does it cost to fix?▼
Repair costs typically range from $150 to $400, including parts and labor. The auxiliary pump itself costs $100-$250, plus diagnostic time.
5. Commonly Affected Makes
This is a generic OBD2 code, potentially applicable to any manufacturer using this diagnostic standard.