P0C79

What it means

P0C79 means the control unit for the drive motor inverter has seen the high-voltage (DC link) voltage at the inverter go above its permitted limit. This is usually judged from the inverter's own voltage sensing, compared with the battery voltage and the limits set in the control software. The exact trigger threshold and the strategy used in response vary by model and system design.

Symptoms

• Hybrid or electric system warning light, or a general malfunction light, on the dashboard
• Reduced power or limp-home mode
• Loss of drive, or the car shutting down the high-voltage system while driving
• Reduced or no regenerative braking
• Car will not enter ready mode or will not move off
• Other high-voltage or inverter related codes stored alongside

Possible causes

• Fault in the inverter, such as a failed voltage sensing circuit or internal power electronics faultlikely
• High-voltage battery or its contactors or management system not controlling bus voltage correctly, for example an overvoltage condition or a contactor opening during regenerationpossible
• Poor connection or damage in the high-voltage cables or connectors, or in the low-voltage sensing and communication wiring to the inverterpossible
• Fault in the DC-DC converter or another high-voltage component affecting the bus voltagepossible
• Faulty or outdated control unit software or calibration, where the manufacturer has identified this as a known cause for the modelrare
• Failed internal capacitor or discharge circuit in the inverter, allowing the bus voltage to spikerare

How to diagnose

1. Read all stored and pending codes and note the freeze frame data, including vehicle speed, bus voltage, battery voltage and whether the car was accelerating or regenerating at the time
2. Check for related codes covering the high-voltage battery, contactors, DC-DC converter, motor position sensing and communication faults, and deal with those first
3. Look for service bulletins or software updates relevant to the model and the code, as the procedure depends on the vehicle
4. Visually inspect the low-voltage wiring and connectors to the inverter and battery management system for damage, corrosion, water ingress or looseness
5. Only if qualified and following the manufacturer's isolation procedure, inspect the high-voltage cables and connectors for damage, heat marks and secure fitting
6. With live data, compare the voltage reported by the inverter with the voltage reported by the battery system at rest and under load, to see which reading is wrong
7. Check the DC-DC converter and the 12 volt system for correct operation, as low-voltage supply faults can cause false readings
8. Carry out the manufacturer's inverter and high-voltage system tests to confirm whether the inverter or something else is at fault

How to fix

• Repair or replace damaged low-voltage wiring or connectors, or high-voltage connections, using the manufacturer's procedure and a qualified technician
• Replace or repair the inverter if testing shows a sensor or internal power electronics fault
• Repair the high-voltage battery, contactors or battery management system if they are found to be allowing the voltage to go too high
• Replace the DC-DC converter or other faulty high-voltage component if it is confirmed as the cause
• Update the control unit software or calibration only where the manufacturer lists this as the fix for the fault
• Clear the code after the repair and road test to confirm it does not return

Can I keep driving?

Nearby codes

• P0C78GenericHybrid/EV Battery System Precharge Time Too Long
• P0C7AGenericDrive Motor B Inverter Voltage Too High