Trouble Shooting "Shift to P Position When Starting"

2010 Prius with 220,057 miles. Flat surface and all doors closed.

The car has been acting a little sluggish with an occasional "check hybrid system" on display. Thinking it might be a bad or going away cell in the hybrid battery and/or corroded main terminals (symptom in our other two Prius). All cells checked (7.50 to 7.65v) out fine with the exception of a little corrosion on the hybrid battery main terminals. I cleaned off both connections, put everything back together and reconnected the 12v battery which was/is at full charge (12.6).
When I went start, "check hybrid system, "shift to P position when starting" came up on the display along with a display full of lights. No codes on my code reader.

Any ideas?

 

I did not clean the fan in the HV area.
The battery is a couple years old.
Code reader is a U480 CAN OBDII OBD2
I did double check all the connections. They are tight
Bought the car used several years ago. No problems until the "check hybrid system" warning came up a few weeks ago. I was able to clear with the code reader.
No history of accidents. Prior to or since we purchased the vehicle.

 

I ordered one. Great advise. Thank you.

 

ISSUE RESOLVED!! Turns out it was a couple things. Main sensor on Converter/inverter and the Converter/inverter itself. Replaced both and cleared codes. Working great now!

Thank you for you help.