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DTC codes / P0Cxx · Hybrid propulsion / P0C31

P0C31Inverter B Cooling System Performance

P0C31 indicates that the inverter's dedicated cooling system is not performing as expected, meaning the powertrain control module has detected that the coolant flow or heat rejection is insufficient for the inverter's current operating conditions. The computer typically sets this code when the inverter temperature rises above a modeled threshold or when the cooling system's monitor fails to see the expected temperature drop within a calibrated time.

SystemPowertrain
Code typeGeneric (all makes)
SeverityHigh
Fix difficultyProfessional
AreaHybrid propulsion

Symptoms you may notice

  • Illumination of the check engine light or a hybrid/EV-specific warning lamp, often with no other noticeable symptoms
  • Reduced power output or a forced limp mode when the inverter temperature climbs too high
  • Audible cooling fan running excessively or at maximum speed more often than usual
  • Overheating warning message on the dashboard during sustained driving or heavy electrical load
  • In rare cases, a burning smell or visible coolant residue near the inverter assembly

Common causes, most likely first

  1. Low coolant level in the inverter cooling loop due to a slow leak or improper fill procedure
  2. Air trapped in the inverter cooling circuit after service, preventing proper coolant circulation
  3. Failed or weak electric coolant pump that cannot maintain the required flow rate
  4. Blocked or restricted coolant passage inside the inverter cold plate or radiator
  5. Faulty inverter temperature sensor sending inaccurate readings to the control module
  6. Damaged or kinked coolant hose restricting flow to the inverter
  7. Cooling fan failure or poor airflow across the inverter radiator

How to diagnose and fix it

  1. Check the coolant level in the inverter reservoir and top up with the specified coolant if low, then inspect for external leaks
  2. Use a scan tool to command the electric coolant pump on and verify actual coolant flow and pump operation
  3. Perform a cooling system bleed procedure to remove trapped air from the inverter loop
  4. Measure the inverter temperature sensor resistance and compare to specifications, replacing it if out of range
  5. Inspect all coolant hoses and connections for kinks, collapses, or restrictions and repair as needed
  6. Test the cooling fan operation and replace the fan assembly if it fails to run when commanded
  7. If all external components check out, flush the inverter cooling circuit and verify the cold plate is not internally blocked

Before you replace the inverter assembly

  • A significant number of P0C31 cases are caused by air pockets in the cooling system after a routine coolant service, so a proper bleed procedure often resolves the code without any parts replacement.
  • The electric coolant pump can fail intermittently, so a pump that runs during a static test may still be the culprit under real driving conditions; live data logging during a road test is essential.
  • A restricted or clogged inverter cold plate is frequently overlooked because the coolant may appear to flow freely at the reservoir, but internal blockages reduce heat transfer and set the code.
  • The code can be set by a faulty temperature sensor even when the cooling system is functioning perfectly, so always compare sensor readings to actual measured temperatures with an infrared thermometer.
  • On some vehicles, a weak 12-volt battery or poor ground connection can cause the coolant pump to run slower than commanded, triggering the performance code without any mechanical fault.

Can I keep driving with P0C31?

Do not continue driving with this code active, as inverter overheating can lead to sudden power loss or permanent damage to the hybrid/EV drive components. Have the vehicle towed or driven only a very short distance to a repair facility, and avoid any heavy acceleration or sustained high-speed driving.

Tools that help

Tools we recommend are in the Auto Veteran shop on Amazon. As an Amazon Associate we earn from qualifying purchases.

Guidance is general and written for all OBD-II vehicles. Exact tests and values vary by model, so confirm the fault before replacing parts.

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