P0C42: Hybrid/EV Battery Pack Coolant Temperature Sensor A Circuit
What to do
This measures the coolant, not the battery — worth stating because the two get confused and they lead to different repairs. The useful comparison is coolant temperature against cell temperature: cold coolant with hot cells means the coolant has stopped moving, which is a pump or an airlock rather than a battery fault.
What this code means
The sensor measuring the temperature of the coolant flowing through the battery pack has a circuit fault.
This is not one of the battery temperature sensors, and the difference is the most useful thing on this page. Sensors A to P measure the cells. This measures the coolant. They answer different questions: how hot is the battery, and is the cooling actually working.
The diagnosis lives in the gap between the two. A pack under load should show coolant leaving warmer than it arrived, and cells a little warmer than the coolant around them. Cold coolant beside hot cells means flow has stopped — a pump not running, a valve shut, an airlock — and no amount of the cell sensors reading correctly will tell you that on their own. Coolant as hot as the cells means flow is fine and the heat has nowhere to go, which points at the radiator or the outside air rather than at the pack.
So this sensor's value is comparative, and losing it removes a discriminator rather than a reading.
Note that a pack with this code is liquid-cooled: on an air-cooled pack these codes do not exist, which is worth confirming before hunting for a coolant circuit that was never fitted.
Likely causes
Start here
- Faulty engine coolant temperature sensor · moderate
- Corroded or loose connector · easy
Less common
- Low coolant level · easy
- Coolant leak into the sensor connector · 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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