The P0C21: Auxiliary Transmission Fluid Pump Phase U-V-W Circuit Low fault code indicates that one of the phases (U, V, or W) in the auxiliary transmission fluid pump is experiencing lower than expected voltage. This issue can be caused by faulty wiring, poor connections, or problems within the electrical system. Low voltage can result in improper operation of the pump, which may affect transmission cooling and performance.
P0C21 – Quick Overview
| Code | Information |
|---|---|
| Meaning | P0C21: Auxiliary Transmission Fluid Pump Phase U-V-W Circuit Low |
| Is it serious? | Yes, low voltage can cause the pump to malfunction, leading to transmission issues. |
| Possible causes | – Low voltage in the pump phase U, V, or W circuit – Faulty wiring or connections – Electrical system problems |
| How to diagnose? | – Measure voltage in the pump phase U, V, and W circuits – Inspect wiring and connections – Check electrical system for issues |
P0C21 Meaning
The P0C21 code indicates that one of the phases (U, V, or W) in the auxiliary transmission fluid pump is receiving lower than normal voltage. The pump relies on proper voltage to circulate transmission fluid efficiently, which is critical for cooling and smooth operation. Low voltage could be caused by damaged wiring, poor electrical connections, or broader electrical system issues.
Step-by-step diagnostic guide
| Action | Description | Tools Needed |
|---|---|---|
| Check for Other Codes | Use an OBD-II scanner to check for any additional related codes that may provide more context or point to electrical or pump-related issues. | OBD-II Scanner |
| Measure Voltage in the Pump Phase U, V, and W Circuits | Use a multimeter to measure the voltage in the phase U, V, and W circuits of the auxiliary transmission fluid pump. Compare the readings with manufacturer specifications. | Multimeter, Vehicle Manual |
| Inspect Wiring and Connections | Visually inspect the wiring and connectors for signs of damage, corrosion, or loose connections that could cause a low voltage reading. Repair or replace as necessary. | Flashlight, Multimeter, Wiring Diagram |
| Check the Electrical System | Inspect the vehicle’s electrical system for issues such as poor grounding, battery problems, or voltage irregularities that may be contributing to the low voltage in the pump circuits. | Multimeter, Flashlight, Electrical System Tools |
| Test Pump Components | If the wiring and electrical system are functioning properly, test the auxiliary transmission fluid pump components to ensure they are not causing the low voltage condition. | Diagnostic Tool, Pump Testing Equipment |
| Clear the Code and Test Drive | After making necessary repairs, clear the fault code using an OBD-II scanner and perform a test drive to ensure the issue is resolved and the pump is operating properly. | OBD-II Scanner, Vehicle Owner’s Manual |
| Recheck for Codes | After the test drive, re-scan the system to ensure that the P0C21 code does not return. | OBD-II Scanner |
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Hi, I am Juraj “Yuri” Lukacko. I got frustrated by unhelpful and scammy mechanics, so I decided to learn everything about car diagnostics myself. I test dozens of new car diagnostic tools every month along with learning new strategies to fix and customize cars. About Juraj Lukacko (Yuri)