Shift system malfunction

Hello, and thanks in advance for your help. The heat wasn’t working in my 2016 Prius with 156k, and the mechanic said it was the water pump. After replacing the water pump, they said it was a problem with the engine because it wasn’t drawing the coolant into the engine. So after replacing the engine with a used one with 43k, the heat still wasn’t working and the problem is allegedly the heater exchanger and catalytic converter, which are on backorder. So the heat won’t work until those parts are in but the car is allegedly safe to drive. But wait, there’s more: the temperature light started blinking, so I took it back and one of the “new” engine’s gaskets needed replacing. Done, and the same day, I tried to put the car in Park and got the message “Shift system malfunction, apply parking brake securely when parking, see owner’s manual,” and nothing would happen when I pressed Park. Straight back to the mechanic, who says that is probably an issue with the auxiliary battery, to which I call BS because that battery was replaced less than a year ago.

So, wondering if anyone has had the Shift System Malfunction warning, and if it’s truly just a battery issue or something to do with the engine and/or incompetence. Thanks for your help.

 

I'm in for 6 grand at this point. Would never have put that much into such an old car, much as I love her. Thanks for your advice.

 

I've had it since 2018/34k miles. I'm hoping this is a 250k mile car, at least!

 

Thank you!