Auxiliary Transmission Fluid Pump Phase U-V-W Circuit Low
The P0C21 code indicates that the Powertrain Control Module (PCM) has detected a low voltage or resistance condition in the phase U-V-W circuit of the auxiliary transmission fluid pump. This pump is responsible for circulating transmission fluid to cool and lubricate the transmission when the engine is off, such as during start-stop events. A low circuit condition typically points to a wiring issue, a faulty pump motor, or a poor electrical connection.
01. Most Common Causes
- Damaged or corroded wiring in the pump circuit
- Faulty auxiliary transmission fluid pump motor
- Poor electrical connection at the pump or PCM
- Blown fuse or relay for the pump circuit
02. Symptoms You Will Notice
- Transmission overheating or slipping
- Check Engine Light illuminated
- Reduced fuel economy
- Transmission may not engage properly during start-stop
03. Step-by-Step Diagnosis
Step 1: Use a scan tool to confirm the code and check for related codes.
Step 2: Visually inspect the wiring and connectors for damage, corrosion, or loose connections.
Step 3: Test the pump motor resistance and voltage supply using a multimeter.
Step 4: Check the fuse and relay for the auxiliary pump circuit.
Step 5: If all else is okay, replace the auxiliary transmission fluid pump.
04. Commonly Affected Makes
05. Frequently Asked Questions
Is it safe to drive with this code?▼
It is not recommended to drive with this code as it can lead to transmission overheating and damage, especially in stop-and-go traffic.
How much does it cost to fix?▼
The cost typically ranges from $50 to $150 for diagnosis and repair, but can be higher if the pump needs replacement.