What Does P0C21 Mean?
DTC P0C21 indicates that the three-phase motor circuit (U, V, W) of the auxiliary transmission fluid pump is reading abnormally low voltage or current. This low-circuit condition suggests a short to ground in one or more of the pump motor phase wires, a partially shorted motor winding, or a failed low-side driver in the pump controller. The pump may stall, run erratically, or draw excessive current, triggering the controller to shut down.
Safety Warning
A shorted pump circuit can cause the pump to stall, resulting in loss of transmission hydraulic pressure during EV operation. Additionally, a short-to-ground condition can blow fuses or damage the pump controller, escalating the repair scope.
Common Causes
30%
Pump motor phase wire shorted to ground from chafing or damage
25%
Internal motor winding short to ground (insulation breakdown)
20%
Water intrusion into the pump motor or connector causing leakage current
15%
Failed low-side MOSFET in the pump controller (shorted)
10%
Contaminated or corroded connector pins causing low-resistance path
Diagnostic Steps
1
Step 1: Review freeze-frame data for pump motor current draw at the time of fault. An abnormally high current with low-circuit code indicates a short. Check for P0C20 and P0C22 as well.
2
Step 2: Disconnect the pump motor connector and measure insulation resistance between each phase wire and chassis ground. Any reading below 1 MΩ indicates insulation breakdown.
3
Step 3: Measure phase-to-phase resistance at the pump motor connector. All three pairs should be balanced. A significantly lower reading on any pair indicates a shorted winding.
4
Step 4: Inspect the pump motor wiring harness for chafing, especially where it routes near hot exhaust components or sharp metal edges. Check the connector for water intrusion.
5
Step 5: If the motor and wiring test normal, the pump controller low-side MOSFETs may be at fault. Replace the pump controller module.
Estimated Repair Cost
$350 - $2,200
Parts + labor, varies by vehicle and location
DTC P0C21 indicates that the three-phase motor circuit (U, V, W) of the auxiliary transmission fluid pump is reading abnormally low voltage or current. This low-circuit condition suggests a short to ground in one or more of the pump motor phase wires, a partially shorted motor winding, or a failed l...
The most common cause of P0C21 (Auxiliary Transmission Fluid Pump Phase U-V-W Circuit Low) is: Pump motor phase wire shorted to ground from chafing or damage
Typical repair costs for P0C21 range from $350 to $2,200, depending on the vehicle, location, and whether you do it yourself or go to a shop.
A shorted pump circuit can cause the pump to stall, resulting in loss of transmission hydraulic pressure during EV operation. Additionally, a short-to-ground condition can blow fuses or damage the pump controller, escalating the repair scope.
Start by connecting an OBD2 scanner to read the code and any freeze frame data. Then follow the diagnostic steps specific to P0C21 to identify the root cause.
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Quick Info
Category
Powertrain
System
Hybrid/EV Transmission Auxiliary Pump
Difficulty
Type
Generic (SAE)
Recommended Tools
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