P0C31: Inverter B Cooling System Performance
What to do
Check the electronics coolant level first — it is a separate reservoir from the engine's on most vehicles and easy to miss entirely. If the car has recently had any coolant work, say so: air trapped in the circuit is the commonest cause and needs a proper bleed rather than a part. Expect reduced performance under sustained load until it is sorted.
What this code means
The cooling system serving inverter B is not achieving what is asked of it.
This is an outcome code rather than a circuit code, and that distinction runs through the whole cooling group. The pump codes say a pump's circuit is faulty; the temperature sensor codes say a reading is unusable. This one says the temperatures are being measured correctly, the pump is being commanded correctly, and the heat is still not going away.
Which points at flow rather than electrics, and the list is short and mostly cheap: low coolant, air trapped in the circuit after a repair, a blocked or collapsed hose, a radiator packed with debris, or a valve that is not directing coolant where it should. Air is worth naming first because it is the commonest sequel to any coolant work — these circuits are awkward to bleed, and a pocket of air in the wrong place stops flow while everything appears to be running.
When it arrives with no component code beside it, that is the signal to look at flow rather than at parts.
Likely causes
Start here
- Low inverter coolant level · easy
- Air trapped in the inverter cooling circuit · moderate
- Faulty inverter coolant pump · moderate
Less common
- Restricted coolant flow or blocked passage · moderate
- Blocked or restricted radiator · moderate
Symptoms
Components involved
Information, not instruction
CarGeek is a reference, not a service manual. Its pages are written from public sources and from what other people have found, never from a manufacturer's own service information, so they can be wrong or incomplete for your particular vehicle. Some of what they describe carries real risk: high voltage, fuel under pressure, stored energy, hot and moving parts.
Have anything you intend to act on checked by a trained person against the manufacturer's own procedure for your vehicle. Nothing here replaces that procedure.
Community experience
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