Auxiliary Transmission Fluid Pump Phase U-V-W Circuit/Open
The P0C20 code indicates an open or malfunction in the phase U-V-W circuit of the auxiliary transmission fluid pump. This pump is typically found in hybrid or electric vehicles to circulate transmission fluid when the main engine is off. The circuit monitors the three-phase voltage; an open circuit can prevent the pump from operating, leading to inadequate fluid circulation and potential transmission overheating or damage.
1. Symptoms You Will Notice
- Transmission overheating warning light
- Reduced fuel economy or hybrid system performance
- Transmission slipping or harsh shifting
- Vehicle may enter limp mode or fail to start
2. Most Common Causes
- Faulty transmission auxiliary fluid pump inverter
- Open or damaged wiring in phases U, V, or W
- Corroded, loose, or disconnected auxiliary pump motor connector
- Internal auxiliary pump motor failure (open winding)
- Defective transmission control module (TCM) or hybrid control module
- Blown or damaged fuse or relay supplying the pump inverter
3. Step-by-Step Diagnosis
Step 1: Use a scan tool to confirm the code and check for related codes.
Step 2: Visually inspect the pump wiring and connectors for damage, corrosion, or loose connections.
Step 3: Measure resistance and voltage at the pump connector per manufacturer specifications.
Step 4: Test the pump operation by applying direct power and ground (if safe).
Step 5: If wiring and pump are okay, test the control module or relay.
4. Frequently Asked Questions
Is it safe to drive with this code?▼
No, driving with this code can cause transmission overheating and damage. It is best to have it diagnosed and repaired promptly.
How much does it cost to fix?▼
The cost typically ranges from $50 to $150 for diagnosis and minor repairs like wiring fixes, but can be higher if the pump or control module needs replacement.
5. Commonly Affected Makes
This is a generic OBD2 code, potentially applicable to any manufacturer using this diagnostic standard.