P3125-277 fault code

So the Prius came up with the overheat temperature symbol within the cluster beside the speedo. Then a nice car symbol thereafter. Fortunately my good wife stopped the car ASAP and turned it off. She found it lacked power going up hill and then the dashboard lit up.

So towed it home and pulled the codes. P3125-277 Inverter problem.

Looking this up I'm getting the notes that this is:
277 – Sensor Malfunction. Motor inverter temperature sensor performance problem

Sensor malfunction?
I'm assuming the same goes into a failsafe mode hence the power loss?
And anyone got a step for a hint which sensor I'm looking to check?

Thanks

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Quick check this morning before going to work and found that there is coolant in the inverter reservoir so no leaks! I haven't checked to see if the coolant is moving so I'll do that later.

Might be the pump?

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So I've removed the pump as it ran for a few seconds then stopped. Checked there was 12v present so the relay is doing its job.

Got the pump on the bench and applied 12v to it. Again it runs for a few seconds then stops. But I have to wait an undetermined time between tries.

Impeller is free so nothing sticking it.

Is this a good enough test to suggest the pump is faulty?

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Update: picked up another gen1 pump from a breakers. It was worth a punt for £20. Tested it and it runs with a kick. You can actually feel air being forced out.
Fitted it and taken the car a run. All appears to be good again.

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