Water pump is dying?

P0C73 concerns the cooling system for the electronics, not for the engine.

 

Is there any reason to worry about the engine water pump? 96℃ would not concern me as an engine coolant temperature; it is but 1 Celsius degree above switching on the cooling fans, and well below the temperature to light the overheat light.

 

The screenshot of your P0C73-776 freeze frame doesn't show the inverter cooling system pump RPM. I'm sure it's there, scrolled further down in the freeze frame, I just don't see it in the part you posted.

It's funny about the fortune cookies for trouble codes. For codes in the P1 range (left up to the manufacturer to define), the bad news is you have to be very careful to look in a repair manual for your own make/model of car, or you could google up a completely wrong meaning for that code for some completely different car. But the good news is, when you find the code details for your car, they're completely specific, because Toyota wrote them.

For codes in the P0 range, which have standard definitions assigned by SAE, it's kind of the other way around. The good news is, they're safer to google for, because whatever you come up with should still tell you the SAE standard meaning. The bad news is, the fortune cookie (like Motor Electronics Coolant Pump "A" Control Performance) will be something generic that was cooked up at SAE, not something more Prius-specific the way Toyota might have written it if it were up to them.



So you kind of still have to look in a Toyota repair manual to see exactly what Toyota means when a Prius shows the SAE standard Motor Electronics Coolant Pump "A" Control Performance code.

Toyota Service Information and Where To Find It | PriusChat

It turns out a Prius has two electric "motors" (MG1 and MG2) and a box of electronics to power them, and there's one coolant loop that cools the electronics and both motors, and one pump that moves that coolant, and we would ordinarily just call it the inverter cooling pump, but that's what Toyota lined up with the SAE standard Motor Electronics Coolant Pump "A".

The code can be set if the pump RPM intermittently isn't right, even if it seems ok when you're looking. (That's why I wonder what the RPM was in your P0C73 freeze frame. Also, of course, the target RPM—what the ECU wanted it to be.)



The way the pump is controlled, it has battery voltage available at +BWP, and the Power Management Control ECU sends it a pulse train on IWP where the pulse widths tell it how fast to turn, and its RPM can be read back by the ECU as pulses on NIWP. Old-time readers can recognize that as the same way the engine water pump is controlled (only the engine pump is controlled by the ECM, not by the Power Management Control ECU), so the same principles discussed there apply.



I snarfed that diagram from a 2015 Gen 3 liftback repair manual, so you might take the connector/pin numbers with a grain of salt; better to look those up in the wiring diagram for your own car.

 

How regularly has the coolant been changed?

The coolant contains anticorrosion additives that lose their effectiveness over time. If the coolant gets old enough, it can start corroding the aluminum inside the inverter, making a weird kind of jelly.

 

''after releasing the gas pedal the engine rpm goes up and only then down''


That could be that the hybrid battery is hot, and the computer is trying to use it as less as possible, also the computer will use some of the hybrid battery charge by turning the ICE, revving it up when going downhills and coming to a stop, especially when the hybrid battery modules are not doing so good due to the Prius C not being so good at cooling its own hybrid battery.

 

Overheating of the battery? you can avoid it by using the A/C as much as you can, maybe get an OBD2 scanner and use it to turn on the fan as i do. The Prius C does not cools the hybrid battery so well on its own even with the A/C on, unless you turn it at full blast then the battery cooling fan will turn on at full blast.

 

I would at a minimum, attempt to flush-out the inverter cooling system. You've obviously got some crud in there, that needs to be cleared out. Constant overheating of electronic components will eventually kill them.
Have you checked out and cleaned your battery cooling system & duct-work?

Just my 2 cents....

 

You said that the ICE was running and reving up when coming to a stop or going downhill, 3 things can cause that, a hybrid battery that whose modules are old and can not hold charge, hence the ECU uses the stored energy since since the battery can not hold much. Hybrid battery is getting hot and the ECU avoids charging or discharging it as much as possible so it cools down. Hybrid cooling fan is clogged, can not cool the battery.

 

How hot your motor generators got? i think the motor generators on my 2012 Prius C got to 112 degrees C on a long trip with quite a few uphills.

 

Yes, MG1 & MG2 is cooled by the ATF. Control electronics - inverter is cooled by the inverter coolant and that cooling loop.