P0E84: Auxiliary Cooling System Coolant Temperature Sensor A Circuit High
What to do
Expect reduced power under sustained load and slower rapid charging. If coolant is being topped up, make sure it is the right circuit and the right specification — an electric car has a separate low-temperature loop for the battery and electronics, and it does not share the engine's coolant.
What this code means
The circuit for the auxiliary cooling system's coolant temperature sensor is faulty.
An electric car has more than one cooling circuit, and "auxiliary" is the standard's word for the low-temperature loop that is not the engine's.
On a hybrid there are at least two, at genuinely different temperatures. The engine runs its coolant at around ninety degrees. The power electronics and the battery want far cooler than that — an inverter is happiest well below sixty, and a lithium pack below thirty — so they get their own loop, their own pump and their own radiator, usually mounted ahead of the engine's.
Mixing the two up is the mistake this page exists to prevent. A technician who tops up the engine's coolant and considers the cooling system checked has not touched the circuit this code is about, and the two often use different coolant specifications — putting the wrong one in an electronics loop is a real and expensive error, because these circuits run alongside high-voltage components and their coolant is chosen for conductivity as well as heat capacity.
Without this measurement the car cannot manage the loop, so it defaults to running pumps and fans hard, or to derating early. The visible result is usually reduced power and slower rapid charging rather than any temperature warning.
At or above the upper limit, usually an open circuit.
Likely causes
Start here
- Faulty inverter coolant temperature sensor · specialist
- Corroded or loose connector · specialist
- Open circuit in sensor wiring · specialist
Less common
- Damaged wiring harness · specialist
- Failed sensor reference voltage supply · specialist
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.
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