P0C71 – Hybrid Battery Temperature Sensor Correlation “D”/”E

P0C71 Possible Causes

  • Faulty hybrid battery thermistor
  • Faulty battery intelligent unit
  • Hybrid battery thermistor harness is open or shorted
  • Poor electrical connection in hybrid battery thermistor circuit

What Does It Mean?

How to Fix Code P0C71

Check the “possible causes” listed above. Visually inspect the related wiring harness and connectors. Check for damaged components and look for broken, bent, ejected, or corroded connector pins.

P0C71 Possible Symptoms

Check engine light illuminated (or service engine soon warning light)

P0C71 Meaning

The battery temperature sensors are provided at 3 HV battery locations. The thermistor resistance, built into each battery temperature sensor, varies according to HV battery temperature changes. The lower the battery temperature, the higher the thermistor resistance. Conversely, the higher the temperature, the lower the resistance. The battery intelligent unit uses the battery temperature sensors to detect HV battery temperature and sends the detected value to the power management control ECU. Based on this detection result, the power management control ECU controls the radiator fan. (The radiator fan starts when the HV battery temperature exceeds a predetermined level.)

P0C72 – Correlation of Hybrid Battery Temperature Sensor “E”/”F

P0C72 Possible Causes

  • Faulty hybrid battery thermistor
  • Faulty battery intelligence unit
  • Hybrid battery thermistor harness is open or shorted
  • Poor electrical connection in hybrid battery thermistor circuit

What Does It Mean?

How to Fix P0C72 Code

Check the “possible causes” listed above. Visually inspect the related wiring harness and connectors. Check for damaged components and look for broken, bent, pushed out, or corroded connector pins.

P0C72 Possible Symptoms

Check engine light illuminated (or service engine soon warning light)

P0C72 Meaning

Battery temperature sensors are provided at 3 HV battery locations. The thermistor resistance, built into each battery temperature sensor, varies according to HV battery temperature changes. The lower the battery temperature, the higher the thermistor resistance. Conversely, the higher the temperature, the lower the resistance. The battery intelligence unit uses the battery temperature sensors to detect HV battery temperature and sends the detected value to the power management control ECU. Based on this detection result, the power management control ECU controls the radiator fan. (The radiator fan starts when the HV battery temperature exceeds a predetermined level.)

P0C73 – Control Performance of Engine Electronics Coolant Pump ‘A

P0C73 Possible Causes

  • Faulty inverter cooling system
  • Inverter cooling system wiring harness is open or short-circuited
  • Poor electrical connection of the inverter cooling system circuit
  • Inverter water pump assembly
  • Power management control ECU

What Does It Mean?

How to Fix Code P0C73

Check the “Possible Causes” listed above. Visually inspect the wiring harness and related connectors. Check for damaged components and look for broken, bent, pushed out, or corroded connector pins.

P0C73 Symptoms

Possible

Engine light on (or service engine soon warning light)

P0C73 Meaning

The inverter water pump assembly transmits rotation speed information to the power management control ECU (HV CPU). The power management control ECU (HV CPU) monitors the speed and detects malfunctions.

P0C74 – Electronic Engine Control “B” Cooling Pump Performance

P0C74 Possible Causes

  • Faulty inverter cooling system
  • Inverter cooling system wiring harness is open or short-circuited
  • Poor electrical connection of the inverter cooling system circuit
  • Inverter water pump assembly
  • Power management control ECU

What Does It Mean?

How to Fix Code P0C74

Check the “possible causes” listed above. Visually inspect the wiring harness and related connectors. Check for damaged components and look for broken, bent, pushed out, or corroded connector pins.

P0C74 Possible Symptoms

Check engine light on (or service engine soon warning light)

P0C74 Meaning

The inverter water pump assembly transmits rotation speed information to the power management control ECU (HV CPU). The power management control ECU (HV CPU) monitors the speed and detects malfunctions.

P0C75 – Hybrid Battery System Discharge Time Too Short

P0C75 Possible Causes

  • Faulty inverter with converter assembly
  • Inverter with converter wiring harness open or short-circuited
  • Inverter with converter assembly circuit poor electrical connection
  • Faulty power management control ECU

What Does It Mean?

How to Fix Code P0C75

Check the “Possible Causes” listed above. Visually inspect the wiring harness and related connectors. Check for damaged components and look for broken, bent, pushed out, or corroded connector pins.

P0C75 Possible Symptoms

Check Engine Light illuminated (or Service Engine Soon warning light)

P0C75 Meaning

The inverter contains a three-phase bridge circuit, which consists of six power transistors (IGBT) for MG1 and MG2 each. The inverter converts high voltage DC current from the HV battery to three-phase AC current for the generator (MG1) and motor (MG2); it also converts three-phase AC current supplied by the generator (MG1) and motor (MG2) to DC current for the HV battery. The MG ECU controls the operation of the power transistors (IGBT). The inverter transmits necessary control information, such as current and voltage, to the MG ECU.

P0C76 – Hybrid Battery System Discharge Time Too Long

P0C76 Possible Causes

  • Faulty inverter with converter assembly
  • Inverter with converter harness is open or short-circuited
  • Inverter with converter assembly circuit poor electrical connection
  • Faulty power management control ECU

What Does It Mean?

How to Fix Code P0C76

Check the “Possible Causes” listed above. Visually inspect the related wiring harness and connectors. Check for damaged components and look for broken, bent, pushed out, or corroded connector pins.

P0C76 Possible Symptoms

Check engine light on (or service engine soon warning light)

P0C76 Meaning

The inverter contains a three-phase bridge circuit, which consists of six power transistors (IGBT) for MG1 and MG2 each. The inverter converts high voltage DC current from the HV battery to three-phase AC current for the generator (MG1) and motor (MG2); it also converts three-phase AC current supplied by the generator (MG1) and motor (MG2) to DC current for the HV battery. The MG ECU controls the operation of the power transistors (IGBT). The inverter transmits necessary control information, such as current and voltage, to the MG ECU.

P0C77 – Hybrid Battery System Precharge Time Too Short

P0C77 Possible Causes

  • Faulty inverter with converter assembly
  • Inverter with converter harness is open or short-circuited
  • Inverter with converter assembly circuit poor electrical connection
  • Faulty power management control ECU

What Does It Mean?

How to Fix Code P0C77

Check the “possible causes” listed above. Visually inspect the wiring harness and related connectors. Check for damaged components and look for broken, bent, pushed out, or corroded connector pins.

P0C77 Possible Symptoms

Check engine light on (or service engine soon warning light)

P0C77 Meaning

The inverter contains a three-phase bridge circuit, which consists of six power transistors (IGBT) for MG1 and MG2 each. The inverter converts high voltage DC current from the HV battery to three-phase AC current for the generator (MG1) and motor (MG2); it also converts three-phase AC current supplied by the generator (MG1) and motor (MG2) to DC current for the HV battery. The MG ECU controls the operation of the power transistors (IGBT). The inverter transmits necessary control information, such as current and voltage, to the MG ECU…

P0C78 – Hybrid Battery System Precharge Time Too Long

Meaning of Code P0C78

The system detects a malfunction in the inverter-converter or its control circuit. This critical component:

  1. Converts direct current (DC) from the HV battery into alternating current (AC) to power the electric motors (MG2 = traction, MG1 = generation).

  2. Converts the AC generated by the motors (during energy recovery) into DC to recharge the HV battery.

  3. Uses power transistors (IGBT) controlled by the MG ECU (Motor Generator Control Unit).

Possible Causes (in order of probability)

  1. 🔌 Electrical problem in the wiring harness or connectors:

    • Cut wires, short circuits, loose, oxidized, or damaged connectors.

    • Target: Harness between the inverter, HV battery, and MG ECU.

  2. ⚡ Faulty Inverter-Converter:

    • Internal failure (e.g., blown IGBTs, faulty control circuit).

  3. 💻 MG ECU Failure:

    • The control unit that operates the inverter is at fault (less common than the inverter itself).

  4. 🔋 High Voltage (HV) Battery Issue:

    • Unstable voltage, defective cells (may trigger secondary errors).

Common Symptoms

  • 🚨 Check Engine Light or hybrid/EV error light illuminated.

  • ⚠️ Limp Mode: Reduced power, limited speed.

  • 🔌 Inability to start or switch to “Ready” mode.

  • 🔋 Reduced electric range or abnormal charging behavior.

Repair Procedure (⚠️ High Voltage Danger!)

DO NOT attempt this without specific electric/hybrid vehicle training!
The HV battery (200-800V) can cause fatal electrocution. Consult a professional.

Diagnostic Steps (by a qualified technician):

  1. 🔒 HV Safety Lockout:

    • Deactivate the HV battery according to manufacturer procedure (insulating gloves, disconnecting the service plug).

    • Wait the prescribed time for capacitor discharge.

  2. 🔍 Thorough Visual Inspection:

    • Wiring/Connectors: Look for burns, torn wires, oxidation, bent/broken/corroded pins (especially on the inverter, HV battery, MG ECU).

    • Physical Components: Signs of overheating, coolant leaks (if liquid-cooled).

  3. 📡 Electrical Tests (with specific tools – DMM, oscilloscope):

    • Continuity and resistance of suspected wiring.

    • Supply and reference voltages to/from the MG ECU and inverter.

    • Control signals between the MG ECU and inverter.

  4. 💻 Advanced Software Diagnostics:

    • Use a dedicated scanner (e.g., Techstream for Toyota) to:

      • Read related fault codes (P0Cxx).

      • Check real-time data (HV voltages, temperatures, internal inverter error signals).

      • Perform active tests on the MG ECU and inverter (if possible).

  5. 🧪 Component Testing:

    • Inverter: Often replaced as a unit if faulty (complex bench testing).

    • MG ECU: Typically tested by elimination or controlled replacement.

Possible Repairs:

  • 🧰 Wiring/Connector Repair: Contact cleaning, pin replacement, wire repair, connector replacement.

  • 🔄 Inverter-Converter Replacement: Common solution if faulty (expensive – critical part).

  • 💾 MG ECU Replacement: If diagnosed as faulty (reprogramming required).

  • 🔋 HV Battery Repair/Replacement: If the source of the problem.

Key Recommendations

  1. 🚫 Do not drive with this code: Risk of total breakdown or severe damage (e.g., inverter overheating).

  2. 🔧 Consult a hybrid/EV specialist: Dealerships or specialized workshops have:

    • Safety equipment.

    • Specific diagnostic tools.

    • Technical expertise.

  3. 💸 Expect high cost: The inverter or MG ECU are among the most expensive parts of a hybrid/electric vehicle.

In summary: P0C78 indicates a critical problem in the electrical heart of your vehicle. Its diagnosis and repair require expertise and extreme precautions. Do not delay in having your vehicle inspected by a professional equipped for high voltage systems.

P0C79 – Drive Motor ‘A’ Inverter Voltage Too High

P0C79 Possible Causes

  • Drive Motor “A” Inverter faulty
  • Drive Motor “A” Inverter harness is open or shorted
  • Poor electrical connection in Drive Motor “A” Inverter circuit

What Does It Mean?

How to Fix Code P0C79

Check the “Possible Causes” listed above. Visually inspect the related wiring harness and connectors. Check for damaged components and look for broken, bent, pushed out, or corroded connector pins.

P0C79 Symptoms

Possible

Engine Light On (or Service Engine Soon warning light)

P0C79 Meaning

The Engine Control Module (ECM) monitors the Drive Motor “A” Inverter. The ECM sets the OBDII code when the Drive Motor “A” Inverter does not meet factory specifications.

P0C7A – Drive Motor ‘B’ Inverter Voltage Too High

P0C7A Possible Causes

  • Drive Motor “A” Inverter faulty
  • Drive Motor “A” Inverter harness is open or shorted
  • Poor electrical connection in Drive Motor “A” Inverter circuit

What Does It Mean?

How to Fix Code P0C7A

Check the “Possible Causes” listed above. Visually inspect the related wiring harness and connectors. Check for damaged components and look for broken, bent, pushed out, or corroded connector pins.

P0C7A Possible Symptoms

Engine Light On (or Service Engine Soon warning light)

P0C7A Meaning

The Engine Control Module (ECM) monitors the drive motor “B” inverter. The ECM sets the OBDII code when the drive motor “B” inverter does not meet factory specifications.