P0C09: Drive Motor B Phase U-V-W Circuit Low

P0C09 marks a U, V or W phase line of drive motor B that is pulled low, for example by a short to the negative high-voltage rail, to ground or by a power switch that conducts when it should not. It is the motor B twin of P0C06; the siblings P0C08 and P0C0A cover an open phase and a phase stuck high. No manufacturer documents P0C09 publicly, and GM's hybrid tables use only P0C08 for motor B phase circuits. CARB requires circuit fault monitoring of hybrid components. A low phase usually causes overcurrent too, so P0C04 or insulation codes often accompany it. The motor controller switches motor B off; the vehicle continues with reduced power and a warning lamp.

Symptoms

• Motor B shut down with warning lamp
• Overcurrent or insulation codes stored at the same time

Most likely causes

• Phase cable or winding of motor B shorted to ground or housing
• Shorted low-side power switch in the motor B inverter
• Insulation damage from heat, moisture or chafing

What to check, in order

Start with the simplest. You can tick off each step.

1. Read the codes with a hybrid-compatible scanner and write down the freeze frame data
2. With the connector unplugged, check whether the signal wire is making contact with ground.
3. Make sure the battery cooling air intake is clean
4. Check the 12 V auxiliary battery, which can also trigger warnings
5. Leave any high-voltage measurements to a specialized repair shop

Good to know

• It can trigger limp mode.
• It affects emissions.

What each character of P0C09 means

• P: Powertrain: engine and transmission
• 0: Generic code: same on every make
• C: Subsystem depends on the code range
• 09: Specific fault in that system

Frequently asked questions

Can I keep driving with code P0C09?

If the car still runs, drive as little as possible and go to a specialized repair shop. Do not touch the orange cables: they carry high voltage.

What causes code P0C09?

The most common causes are: phase cable or winding of motor B shorted to ground or housing; shorted low-side power switch in the motor B inverter; insulation damage from heat, moisture or chafing.

Related codes

• P0C08Drive Motor B Phase U-V-W Circuit/Open
• P0C0ADrive Motor B Phase U-V-W Circuit High
• P0C06Drive Motor A Phase U-V-W Circuit Low
• P0C04Drive Motor B Current High
• P0BFEDrive Motor B Phase U-V-W Current Sensor Correlation

Sources

• law.cornell.edu: 13 CCR 1968.2
• gsi.ext.gm.com: 12OBDG06 Hybrid Diagnostics

Values and procedures vary by manufacturer. If it affects brakes, steering, fuel or high voltage, see a professional. Updated: 2026-11-04. How we build these pages.