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, especially during start-stop events or when the engine is off. A low circuit condition typically points to a short to ground, open circuit, or a failing pump motor. This can lead to inadequate transmission cooling and potential damage.
1. Symptoms You Will Notice
- Transmission overheating
- Check Engine Light illuminated
- Reduced transmission performance or limp mode
- Unusual noises from the transmission area
2. Most Common Causes
- Faulty auxiliary transmission fluid pump motor
- Short to ground or open in the pump wiring harness
- Damaged connector or poor electrical connection
- Failed PCM (rare)
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: Using a multimeter, measure resistance between pump phases U, V, W; compare to specifications (typically 0.5-2 ohms).
Step 4: Check for voltage at the pump connector with the engine running; should see battery voltage.
Step 5: If wiring and voltage are OK, test the pump motor by applying direct battery power; if no operation, replace pump.
4. Frequently Asked Questions
Is it safe to drive with this code?▼
It is not recommended. Driving with a failing auxiliary pump can cause transmission overheating and lead to costly internal damage.
How much does it cost to fix?▼
Repair costs typically range from $150 to $400, including parts and labor. The pump itself costs $100-$250, and labor is about $50-$150.
5. Commonly Affected Makes
This is a generic OBD2 code, potentially applicable to any manufacturer using this diagnostic standard.